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"""This test makes sure that the forces returned from a
SinglePointCalculator are immutable. Previously, successive calls to
atoms.get_forces(apply_constraint=x), with x alternating between True and
False, would get locked into the constrained variation."""
from ase.build import fcc111
from ase.calculators.emt import EMT
from ase.io import read
from ase.constraints import FixAtoms
def check_forces():
"""Makes sure the unconstrained forces stay that way."""
forces = atoms.get_forces(apply_constraint=False)
funconstrained = float(forces[0, 0])
forces = atoms.get_forces(apply_constraint=True)
forces = atoms.get_forces(apply_constraint=False)
funconstrained2 = float(forces[0, 0])
assert funconstrained2 == funconstrained
atoms = fcc111('Cu', (2, 2, 1), vacuum=10.)
atoms[0].x += 0.2
atoms.set_constraint(FixAtoms(indices=[atom.index for atom in atoms]))
# First run the tes with EMT and save a force component.
atoms.set_calculator(EMT())
check_forces()
f = float(atoms.get_forces(apply_constraint=False)[0, 0])
# Save and reload with a SinglePointCalculator.
atoms.write('singlepointtest.traj')
atoms = read('singlepointtest.traj')
check_forces()
# Manually change a value.
forces = atoms.get_forces(apply_constraint=False)
forces[0, 0] = 42.
forces = atoms.get_forces(apply_constraint=False)
assert forces[0, 0] == f
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