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737 lines
29 KiB
Python
737 lines
29 KiB
Python
"""
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basinhopping: The basinhopping global optimization algorithm
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"""
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from __future__ import division, print_function, absolute_import
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import numpy as np
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import math
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from numpy import cos, sin
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import scipy.optimize
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from scipy._lib._util import check_random_state
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__all__ = ['basinhopping']
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class Storage(object):
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"""
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Class used to store the lowest energy structure
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"""
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def __init__(self, minres):
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self._add(minres)
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def _add(self, minres):
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self.minres = minres
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self.minres.x = np.copy(minres.x)
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def update(self, minres):
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if minres.fun < self.minres.fun:
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self._add(minres)
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return True
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else:
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return False
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def get_lowest(self):
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return self.minres
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class BasinHoppingRunner(object):
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"""This class implements the core of the basinhopping algorithm.
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x0 : ndarray
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The starting coordinates.
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minimizer : callable
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The local minimizer, with signature ``result = minimizer(x)``.
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The return value is an `optimize.OptimizeResult` object.
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step_taking : callable
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This function displaces the coordinates randomly. Signature should
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be ``x_new = step_taking(x)``. Note that `x` may be modified in-place.
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accept_tests : list of callables
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Each test is passed the kwargs `f_new`, `x_new`, `f_old` and
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`x_old`. These tests will be used to judge whether or not to accept
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the step. The acceptable return values are True, False, or ``"force
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accept"``. If any of the tests return False then the step is rejected.
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If ``"force accept"``, then this will override any other tests in
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order to accept the step. This can be used, for example, to forcefully
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escape from a local minimum that ``basinhopping`` is trapped in.
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disp : bool, optional
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Display status messages.
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"""
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def __init__(self, x0, minimizer, step_taking, accept_tests, disp=False):
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self.x = np.copy(x0)
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self.minimizer = minimizer
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self.step_taking = step_taking
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self.accept_tests = accept_tests
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self.disp = disp
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self.nstep = 0
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# initialize return object
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self.res = scipy.optimize.OptimizeResult()
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self.res.minimization_failures = 0
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# do initial minimization
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minres = minimizer(self.x)
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if not minres.success:
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self.res.minimization_failures += 1
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if self.disp:
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print("warning: basinhopping: local minimization failure")
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self.x = np.copy(minres.x)
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self.energy = minres.fun
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if self.disp:
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print("basinhopping step %d: f %g" % (self.nstep, self.energy))
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# initialize storage class
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self.storage = Storage(minres)
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if hasattr(minres, "nfev"):
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self.res.nfev = minres.nfev
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if hasattr(minres, "njev"):
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self.res.njev = minres.njev
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if hasattr(minres, "nhev"):
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self.res.nhev = minres.nhev
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def _monte_carlo_step(self):
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"""Do one Monte Carlo iteration
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Randomly displace the coordinates, minimize, and decide whether
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or not to accept the new coordinates.
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"""
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# Take a random step. Make a copy of x because the step_taking
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# algorithm might change x in place
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x_after_step = np.copy(self.x)
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x_after_step = self.step_taking(x_after_step)
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# do a local minimization
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minres = self.minimizer(x_after_step)
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x_after_quench = minres.x
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energy_after_quench = minres.fun
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if not minres.success:
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self.res.minimization_failures += 1
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if self.disp:
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print("warning: basinhopping: local minimization failure")
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if hasattr(minres, "nfev"):
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self.res.nfev += minres.nfev
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if hasattr(minres, "njev"):
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self.res.njev += minres.njev
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if hasattr(minres, "nhev"):
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self.res.nhev += minres.nhev
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# accept the move based on self.accept_tests. If any test is False,
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# then reject the step. If any test returns the special string
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# 'force accept', then accept the step regardless. This can be used
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# to forcefully escape from a local minimum if normal basin hopping
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# steps are not sufficient.
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accept = True
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for test in self.accept_tests:
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testres = test(f_new=energy_after_quench, x_new=x_after_quench,
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f_old=self.energy, x_old=self.x)
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if testres == 'force accept':
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accept = True
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break
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elif testres is None:
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raise ValueError("accept_tests must return True, False, or "
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"'force accept'")
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elif not testres:
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accept = False
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# Report the result of the acceptance test to the take step class.
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# This is for adaptive step taking
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if hasattr(self.step_taking, "report"):
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self.step_taking.report(accept, f_new=energy_after_quench,
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x_new=x_after_quench, f_old=self.energy,
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x_old=self.x)
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return accept, minres
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def one_cycle(self):
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"""Do one cycle of the basinhopping algorithm
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"""
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self.nstep += 1
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new_global_min = False
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accept, minres = self._monte_carlo_step()
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if accept:
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self.energy = minres.fun
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self.x = np.copy(minres.x)
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new_global_min = self.storage.update(minres)
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# print some information
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if self.disp:
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self.print_report(minres.fun, accept)
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if new_global_min:
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print("found new global minimum on step %d with function"
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" value %g" % (self.nstep, self.energy))
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# save some variables as BasinHoppingRunner attributes
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self.xtrial = minres.x
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self.energy_trial = minres.fun
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self.accept = accept
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return new_global_min
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def print_report(self, energy_trial, accept):
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"""print a status update"""
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minres = self.storage.get_lowest()
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print("basinhopping step %d: f %g trial_f %g accepted %d "
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" lowest_f %g" % (self.nstep, self.energy, energy_trial,
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accept, minres.fun))
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class AdaptiveStepsize(object):
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"""
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Class to implement adaptive stepsize.
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This class wraps the step taking class and modifies the stepsize to
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ensure the true acceptance rate is as close as possible to the target.
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Parameters
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----------
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takestep : callable
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The step taking routine. Must contain modifiable attribute
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takestep.stepsize
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accept_rate : float, optional
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The target step acceptance rate
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interval : int, optional
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Interval for how often to update the stepsize
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factor : float, optional
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The step size is multiplied or divided by this factor upon each
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update.
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verbose : bool, optional
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Print information about each update
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"""
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def __init__(self, takestep, accept_rate=0.5, interval=50, factor=0.9,
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verbose=True):
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self.takestep = takestep
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self.target_accept_rate = accept_rate
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self.interval = interval
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self.factor = factor
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self.verbose = verbose
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self.nstep = 0
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self.nstep_tot = 0
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self.naccept = 0
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def __call__(self, x):
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return self.take_step(x)
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def _adjust_step_size(self):
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old_stepsize = self.takestep.stepsize
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accept_rate = float(self.naccept) / self.nstep
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if accept_rate > self.target_accept_rate:
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# We're accepting too many steps. This generally means we're
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# trapped in a basin. Take bigger steps
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self.takestep.stepsize /= self.factor
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else:
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# We're not accepting enough steps. Take smaller steps
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self.takestep.stepsize *= self.factor
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if self.verbose:
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print("adaptive stepsize: acceptance rate %f target %f new "
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"stepsize %g old stepsize %g" % (accept_rate,
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self.target_accept_rate, self.takestep.stepsize,
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old_stepsize))
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def take_step(self, x):
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self.nstep += 1
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self.nstep_tot += 1
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if self.nstep % self.interval == 0:
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self._adjust_step_size()
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return self.takestep(x)
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def report(self, accept, **kwargs):
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"called by basinhopping to report the result of the step"
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if accept:
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self.naccept += 1
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class RandomDisplacement(object):
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"""
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Add a random displacement of maximum size `stepsize` to each coordinate
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Calling this updates `x` in-place.
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Parameters
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----------
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stepsize : float, optional
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Maximum stepsize in any dimension
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random_state : None or `np.random.RandomState` instance, optional
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The random number generator that generates the displacements
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"""
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def __init__(self, stepsize=0.5, random_state=None):
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self.stepsize = stepsize
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self.random_state = check_random_state(random_state)
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def __call__(self, x):
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x += self.random_state.uniform(-self.stepsize, self.stepsize,
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np.shape(x))
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return x
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class MinimizerWrapper(object):
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"""
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wrap a minimizer function as a minimizer class
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"""
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def __init__(self, minimizer, func=None, **kwargs):
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self.minimizer = minimizer
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self.func = func
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self.kwargs = kwargs
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def __call__(self, x0):
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if self.func is None:
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return self.minimizer(x0, **self.kwargs)
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else:
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return self.minimizer(self.func, x0, **self.kwargs)
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class Metropolis(object):
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"""
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Metropolis acceptance criterion
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Parameters
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----------
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T : float
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The "temperature" parameter for the accept or reject criterion.
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random_state : None or `np.random.RandomState` object
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Random number generator used for acceptance test
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"""
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def __init__(self, T, random_state=None):
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# Avoid ZeroDivisionError since "MBH can be regarded as a special case
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# of the BH framework with the Metropolis criterion, where temperature
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# T = 0." (Reject all steps that increase energy.)
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self.beta = 1.0 / T if T != 0 else float('inf')
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self.random_state = check_random_state(random_state)
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def accept_reject(self, energy_new, energy_old):
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"""
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If new energy is lower than old, it will always be accepted.
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If new is higher than old, there is a chance it will be accepted,
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less likely for larger differences.
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"""
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w = math.exp(min(0, -float(energy_new - energy_old) * self.beta))
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rand = self.random_state.rand()
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return w >= rand
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def __call__(self, **kwargs):
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"""
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f_new and f_old are mandatory in kwargs
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"""
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return bool(self.accept_reject(kwargs["f_new"],
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kwargs["f_old"]))
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def basinhopping(func, x0, niter=100, T=1.0, stepsize=0.5,
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minimizer_kwargs=None, take_step=None, accept_test=None,
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callback=None, interval=50, disp=False, niter_success=None,
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seed=None):
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"""
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Find the global minimum of a function using the basin-hopping algorithm
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Basin-hopping is a two-phase method that combines a global stepping
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algorithm with local minimization at each step. Designed to mimic
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the natural process of energy minimization of clusters of atoms, it works
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well for similar problems with "funnel-like, but rugged" energy landscapes
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[5]_.
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As the step-taking, step acceptance, and minimization methods are all
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customizable, this function can also be used to implement other two-phase
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methods.
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Parameters
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----------
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func : callable ``f(x, *args)``
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Function to be optimized. ``args`` can be passed as an optional item
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in the dict ``minimizer_kwargs``
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x0 : array_like
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Initial guess.
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niter : integer, optional
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The number of basin-hopping iterations
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T : float, optional
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The "temperature" parameter for the accept or reject criterion. Higher
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"temperatures" mean that larger jumps in function value will be
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accepted. For best results ``T`` should be comparable to the
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separation (in function value) between local minima.
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stepsize : float, optional
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Maximum step size for use in the random displacement.
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minimizer_kwargs : dict, optional
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Extra keyword arguments to be passed to the local minimizer
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``scipy.optimize.minimize()`` Some important options could be:
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method : str
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The minimization method (e.g. ``"L-BFGS-B"``)
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args : tuple
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Extra arguments passed to the objective function (``func``) and
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its derivatives (Jacobian, Hessian).
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take_step : callable ``take_step(x)``, optional
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Replace the default step-taking routine with this routine. The default
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step-taking routine is a random displacement of the coordinates, but
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other step-taking algorithms may be better for some systems.
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``take_step`` can optionally have the attribute ``take_step.stepsize``.
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If this attribute exists, then ``basinhopping`` will adjust
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``take_step.stepsize`` in order to try to optimize the global minimum
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search.
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accept_test : callable, ``accept_test(f_new=f_new, x_new=x_new, f_old=fold, x_old=x_old)``, optional
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Define a test which will be used to judge whether or not to accept the
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step. This will be used in addition to the Metropolis test based on
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"temperature" ``T``. The acceptable return values are True,
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False, or ``"force accept"``. If any of the tests return False
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then the step is rejected. If the latter, then this will override any
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other tests in order to accept the step. This can be used, for example,
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to forcefully escape from a local minimum that ``basinhopping`` is
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trapped in.
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callback : callable, ``callback(x, f, accept)``, optional
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A callback function which will be called for all minima found. ``x``
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and ``f`` are the coordinates and function value of the trial minimum,
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and ``accept`` is whether or not that minimum was accepted. This can
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be used, for example, to save the lowest N minima found. Also,
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``callback`` can be used to specify a user defined stop criterion by
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optionally returning True to stop the ``basinhopping`` routine.
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interval : integer, optional
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interval for how often to update the ``stepsize``
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disp : bool, optional
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Set to True to print status messages
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niter_success : integer, optional
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Stop the run if the global minimum candidate remains the same for this
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number of iterations.
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seed : int or `np.random.RandomState`, optional
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If `seed` is not specified the `np.RandomState` singleton is used.
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If `seed` is an int, a new `np.random.RandomState` instance is used,
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seeded with seed.
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If `seed` is already a `np.random.RandomState instance`, then that
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`np.random.RandomState` instance is used.
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Specify `seed` for repeatable minimizations. The random numbers
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generated with this seed only affect the default Metropolis
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`accept_test` and the default `take_step`. If you supply your own
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`take_step` and `accept_test`, and these functions use random
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number generation, then those functions are responsible for the state
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of their random number generator.
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Returns
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-------
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res : OptimizeResult
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The optimization result represented as a ``OptimizeResult`` object.
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Important attributes are: ``x`` the solution array, ``fun`` the value
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of the function at the solution, and ``message`` which describes the
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cause of the termination. The ``OptimizeResult`` object returned by the
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selected minimizer at the lowest minimum is also contained within this
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object and can be accessed through the ``lowest_optimization_result``
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attribute. See `OptimizeResult` for a description of other attributes.
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See Also
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--------
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minimize :
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The local minimization function called once for each basinhopping step.
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``minimizer_kwargs`` is passed to this routine.
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Notes
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-----
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Basin-hopping is a stochastic algorithm which attempts to find the global
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minimum of a smooth scalar function of one or more variables [1]_ [2]_ [3]_
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[4]_. The algorithm in its current form was described by David Wales and
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Jonathan Doye [2]_ http://www-wales.ch.cam.ac.uk/.
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The algorithm is iterative with each cycle composed of the following
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features
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1) random perturbation of the coordinates
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2) local minimization
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3) accept or reject the new coordinates based on the minimized function
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value
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The acceptance test used here is the Metropolis criterion of standard Monte
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Carlo algorithms, although there are many other possibilities [3]_.
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This global minimization method has been shown to be extremely efficient
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for a wide variety of problems in physics and chemistry. It is
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particularly useful when the function has many minima separated by large
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barriers. See the Cambridge Cluster Database
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http://www-wales.ch.cam.ac.uk/CCD.html for databases of molecular systems
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that have been optimized primarily using basin-hopping. This database
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includes minimization problems exceeding 300 degrees of freedom.
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See the free software program GMIN (http://www-wales.ch.cam.ac.uk/GMIN) for
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a Fortran implementation of basin-hopping. This implementation has many
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different variations of the procedure described above, including more
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advanced step taking algorithms and alternate acceptance criterion.
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For stochastic global optimization there is no way to determine if the true
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global minimum has actually been found. Instead, as a consistency check,
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the algorithm can be run from a number of different random starting points
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to ensure the lowest minimum found in each example has converged to the
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global minimum. For this reason ``basinhopping`` will by default simply
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run for the number of iterations ``niter`` and return the lowest minimum
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found. It is left to the user to ensure that this is in fact the global
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minimum.
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Choosing ``stepsize``: This is a crucial parameter in ``basinhopping`` and
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depends on the problem being solved. The step is chosen uniformly in the
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region from x0-stepsize to x0+stepsize, in each dimension. Ideally it
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should be comparable to the typical separation (in argument values) between
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local minima of the function being optimized. ``basinhopping`` will, by
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default, adjust ``stepsize`` to find an optimal value, but this may take
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many iterations. You will get quicker results if you set a sensible
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initial value for ``stepsize``.
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Choosing ``T``: The parameter ``T`` is the "temperature" used in the
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Metropolis criterion. Basinhopping steps are always accepted if
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``func(xnew) < func(xold)``. Otherwise, they are accepted with
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probability::
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exp( -(func(xnew) - func(xold)) / T )
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So, for best results, ``T`` should to be comparable to the typical
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difference (in function values) between local minima. (The height of
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"walls" between local minima is irrelevant.)
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If ``T`` is 0, the algorithm becomes Monotonic Basin-Hopping, in which all
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steps that increase energy are rejected.
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.. versionadded:: 0.12.0
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References
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----------
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.. [1] Wales, David J. 2003, Energy Landscapes, Cambridge University Press,
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Cambridge, UK.
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.. [2] Wales, D J, and Doye J P K, Global Optimization by Basin-Hopping and
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the Lowest Energy Structures of Lennard-Jones Clusters Containing up to
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110 Atoms. Journal of Physical Chemistry A, 1997, 101, 5111.
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.. [3] Li, Z. and Scheraga, H. A., Monte Carlo-minimization approach to the
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multiple-minima problem in protein folding, Proc. Natl. Acad. Sci. USA,
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1987, 84, 6611.
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.. [4] Wales, D. J. and Scheraga, H. A., Global optimization of clusters,
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crystals, and biomolecules, Science, 1999, 285, 1368.
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.. [5] Olson, B., Hashmi, I., Molloy, K., and Shehu1, A., Basin Hopping as
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a General and Versatile Optimization Framework for the Characterization
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of Biological Macromolecules, Advances in Artificial Intelligence,
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Volume 2012 (2012), Article ID 674832, :doi:`10.1155/2012/674832`
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Examples
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--------
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The following example is a one-dimensional minimization problem, with many
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local minima superimposed on a parabola.
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>>> from scipy.optimize import basinhopping
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>>> func = lambda x: np.cos(14.5 * x - 0.3) + (x + 0.2) * x
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>>> x0=[1.]
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Basinhopping, internally, uses a local minimization algorithm. We will use
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the parameter ``minimizer_kwargs`` to tell basinhopping which algorithm to
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use and how to set up that minimizer. This parameter will be passed to
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``scipy.optimize.minimize()``.
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>>> minimizer_kwargs = {"method": "BFGS"}
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>>> ret = basinhopping(func, x0, minimizer_kwargs=minimizer_kwargs,
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... niter=200)
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>>> print("global minimum: x = %.4f, f(x0) = %.4f" % (ret.x, ret.fun))
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global minimum: x = -0.1951, f(x0) = -1.0009
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Next consider a two-dimensional minimization problem. Also, this time we
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will use gradient information to significantly speed up the search.
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>>> def func2d(x):
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... f = np.cos(14.5 * x[0] - 0.3) + (x[1] + 0.2) * x[1] + (x[0] +
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... 0.2) * x[0]
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... df = np.zeros(2)
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... df[0] = -14.5 * np.sin(14.5 * x[0] - 0.3) + 2. * x[0] + 0.2
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... df[1] = 2. * x[1] + 0.2
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... return f, df
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We'll also use a different local minimization algorithm. Also we must tell
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the minimizer that our function returns both energy and gradient (jacobian)
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>>> minimizer_kwargs = {"method":"L-BFGS-B", "jac":True}
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>>> x0 = [1.0, 1.0]
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>>> ret = basinhopping(func2d, x0, minimizer_kwargs=minimizer_kwargs,
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... niter=200)
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>>> print("global minimum: x = [%.4f, %.4f], f(x0) = %.4f" % (ret.x[0],
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... ret.x[1],
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... ret.fun))
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global minimum: x = [-0.1951, -0.1000], f(x0) = -1.0109
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Here is an example using a custom step-taking routine. Imagine you want
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the first coordinate to take larger steps than the rest of the coordinates.
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This can be implemented like so:
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>>> class MyTakeStep(object):
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... def __init__(self, stepsize=0.5):
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... self.stepsize = stepsize
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... def __call__(self, x):
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... s = self.stepsize
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... x[0] += np.random.uniform(-2.*s, 2.*s)
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... x[1:] += np.random.uniform(-s, s, x[1:].shape)
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... return x
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Since ``MyTakeStep.stepsize`` exists basinhopping will adjust the magnitude
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of ``stepsize`` to optimize the search. We'll use the same 2-D function as
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before
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>>> mytakestep = MyTakeStep()
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>>> ret = basinhopping(func2d, x0, minimizer_kwargs=minimizer_kwargs,
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... niter=200, take_step=mytakestep)
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>>> print("global minimum: x = [%.4f, %.4f], f(x0) = %.4f" % (ret.x[0],
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... ret.x[1],
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... ret.fun))
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global minimum: x = [-0.1951, -0.1000], f(x0) = -1.0109
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Now let's do an example using a custom callback function which prints the
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value of every minimum found
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>>> def print_fun(x, f, accepted):
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... print("at minimum %.4f accepted %d" % (f, int(accepted)))
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We'll run it for only 10 basinhopping steps this time.
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>>> np.random.seed(1)
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>>> ret = basinhopping(func2d, x0, minimizer_kwargs=minimizer_kwargs,
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... niter=10, callback=print_fun)
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at minimum 0.4159 accepted 1
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at minimum -0.9073 accepted 1
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at minimum -0.1021 accepted 1
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at minimum -0.1021 accepted 1
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at minimum 0.9102 accepted 1
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at minimum 0.9102 accepted 1
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at minimum 2.2945 accepted 0
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at minimum -0.1021 accepted 1
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at minimum -1.0109 accepted 1
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at minimum -1.0109 accepted 1
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The minimum at -1.0109 is actually the global minimum, found already on the
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8th iteration.
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Now let's implement bounds on the problem using a custom ``accept_test``:
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>>> class MyBounds(object):
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... def __init__(self, xmax=[1.1,1.1], xmin=[-1.1,-1.1] ):
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... self.xmax = np.array(xmax)
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... self.xmin = np.array(xmin)
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... def __call__(self, **kwargs):
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... x = kwargs["x_new"]
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... tmax = bool(np.all(x <= self.xmax))
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... tmin = bool(np.all(x >= self.xmin))
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... return tmax and tmin
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>>> mybounds = MyBounds()
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>>> ret = basinhopping(func2d, x0, minimizer_kwargs=minimizer_kwargs,
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... niter=10, accept_test=mybounds)
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"""
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x0 = np.array(x0)
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# set up the np.random.RandomState generator
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rng = check_random_state(seed)
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# set up minimizer
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if minimizer_kwargs is None:
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minimizer_kwargs = dict()
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wrapped_minimizer = MinimizerWrapper(scipy.optimize.minimize, func,
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**minimizer_kwargs)
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# set up step-taking algorithm
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if take_step is not None:
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if not callable(take_step):
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raise TypeError("take_step must be callable")
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# if take_step.stepsize exists then use AdaptiveStepsize to control
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# take_step.stepsize
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if hasattr(take_step, "stepsize"):
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take_step_wrapped = AdaptiveStepsize(take_step, interval=interval,
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verbose=disp)
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else:
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take_step_wrapped = take_step
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else:
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# use default
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displace = RandomDisplacement(stepsize=stepsize, random_state=rng)
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take_step_wrapped = AdaptiveStepsize(displace, interval=interval,
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verbose=disp)
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# set up accept tests
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accept_tests = []
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if accept_test is not None:
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if not callable(accept_test):
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raise TypeError("accept_test must be callable")
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accept_tests = [accept_test]
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# use default
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metropolis = Metropolis(T, random_state=rng)
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accept_tests.append(metropolis)
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if niter_success is None:
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niter_success = niter + 2
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bh = BasinHoppingRunner(x0, wrapped_minimizer, take_step_wrapped,
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accept_tests, disp=disp)
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# start main iteration loop
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count, i = 0, 0
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message = ["requested number of basinhopping iterations completed"
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" successfully"]
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for i in range(niter):
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new_global_min = bh.one_cycle()
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if callable(callback):
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# should we pass a copy of x?
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val = callback(bh.xtrial, bh.energy_trial, bh.accept)
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if val is not None:
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if val:
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message = ["callback function requested stop early by"
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"returning True"]
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break
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count += 1
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if new_global_min:
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count = 0
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elif count > niter_success:
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message = ["success condition satisfied"]
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break
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# prepare return object
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res = bh.res
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res.lowest_optimization_result = bh.storage.get_lowest()
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res.x = np.copy(res.lowest_optimization_result.x)
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res.fun = res.lowest_optimization_result.fun
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res.message = message
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res.nit = i + 1
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return res
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def _test_func2d_nograd(x):
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f = (cos(14.5 * x[0] - 0.3) + (x[1] + 0.2) * x[1] + (x[0] + 0.2) * x[0]
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+ 1.010876184442655)
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return f
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def _test_func2d(x):
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f = (cos(14.5 * x[0] - 0.3) + (x[0] + 0.2) * x[0] + cos(14.5 * x[1] -
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0.3) + (x[1] + 0.2) * x[1] + x[0] * x[1] + 1.963879482144252)
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df = np.zeros(2)
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df[0] = -14.5 * sin(14.5 * x[0] - 0.3) + 2. * x[0] + 0.2 + x[1]
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df[1] = -14.5 * sin(14.5 * x[1] - 0.3) + 2. * x[1] + 0.2 + x[0]
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return f, df
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if __name__ == "__main__":
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print("\n\nminimize a 2d function without gradient")
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# minimum expected at ~[-0.195, -0.1]
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kwargs = {"method": "L-BFGS-B"}
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x0 = np.array([1.0, 1.])
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scipy.optimize.minimize(_test_func2d_nograd, x0, **kwargs)
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ret = basinhopping(_test_func2d_nograd, x0, minimizer_kwargs=kwargs,
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niter=200, disp=False)
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print("minimum expected at func([-0.195, -0.1]) = 0.0")
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print(ret)
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print("\n\ntry a harder 2d problem")
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kwargs = {"method": "L-BFGS-B", "jac": True}
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x0 = np.array([1.0, 1.0])
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ret = basinhopping(_test_func2d, x0, minimizer_kwargs=kwargs, niter=200,
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disp=False)
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print("minimum expected at ~, func([-0.19415263, -0.19415263]) = 0")
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print(ret)
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