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from ase import Atoms
from ase.calculators.emt import EMT
from ase.constraints import FixBondLength
from ase.io import Trajectory
from ase.optimize import BFGS
a = 3.6
b = a / 2
cu = Atoms('Cu2Ag',
positions=[(0, 0, 0),
(b, b, 0),
(a, a, b)],
calculator=EMT())
e0 = cu.get_potential_energy()
print(e0)
d0 = cu.get_distance(0, 1)
cu.set_constraint(FixBondLength(0, 1))
t = Trajectory('cu2ag.traj', 'w', cu)
qn = BFGS(cu)
qn.attach(t.write)
def f(): print(cu.get_distance(0,1))
qn.attach(f)
qn.run(fmax=0.01)
assert abs(cu.get_distance(0, 1) - d0) < 1e-14
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