from ase.calculators.emt import EMT
from ase.optimize import BFGS
from ase.constraints import FixCom
from ase.build import molecule

atoms = molecule('H2O')
atoms.center(vacuum=4)
atoms.set_calculator(EMT())
cold = atoms.get_center_of_mass()
atoms.set_constraint(FixCom())

opt = BFGS(atoms)
opt.run(steps=5)

cnew = atoms.get_center_of_mass()

assert max(abs(cnew - cold)) < 1e-8