def test_dimer():
    from ase import Atom, Atoms
    from ase.calculators.lj import LennardJones
    from ase.constraints import FixBondLength

    dimer = Atoms([Atom('X', (0, 0, 0)),
                   Atom('X', (0, 0, 1))],
                  calculator=LennardJones(),
                  constraint=FixBondLength(0, 1))
    print(dimer.get_forces())
    print(dimer.positions)
    dimer.positions[:] += 0.1
    print(dimer.positions)
    dimer.positions[:, 2] += 5.1
    print(dimer.positions)
    dimer.positions[:] = [(1, 2, 3), (4, 5, 6)]
    print(dimer.positions)
    dimer.set_positions([(1, 2, 3), (4, 5, 6.2)])
    print(dimer.positions)