def test_Ag_Cu100():
    from math import sqrt
    from ase import Atom, Atoms
    from ase.neb import NEB
    from ase.constraints import FixAtoms
    from ase.vibrations import Vibrations
    from ase.calculators.emt import EMT
    from ase.optimize import QuasiNewton, BFGS

    # Distance between Cu atoms on a (100) surface:
    d = 3.6 / sqrt(2)
    initial = Atoms('Cu',
                    positions=[(0, 0, 0)],
                    cell=(d, d, 1.0),
                    pbc=(True, True, False))
    initial *= (2, 2, 1)  # 2x2 (100) surface-cell

    # Approximate height of Ag atom on Cu(100) surfece:
    h0 = 2.0
    initial += Atom('Ag', (d / 2, d / 2, h0))

    # Make band:
    images = [initial.copy() for i in range(6)]
    neb = NEB(images, climb=True)

    # Set constraints and calculator:
    constraint = FixAtoms(range(len(initial) - 1))
    for image in images:
        image.calc = EMT()
        image.set_constraint(constraint)

    # Displace last image:
    images[-1].positions[-1] += (d, 0, 0)

    # Relax height of Ag atom for initial and final states:
    dyn1 = QuasiNewton(images[0])
    dyn1.run(fmax=0.01)
    dyn2 = QuasiNewton(images[-1])
    dyn2.run(fmax=0.01)

    # Interpolate positions between initial and final states:
    neb.interpolate()

    for image in images:
        print(image.positions[-1], image.get_potential_energy())

    with BFGS(neb) as dyn:
        dyn.run(fmax=0.05)

    for image in images:
        print(image.positions[-1], image.get_potential_energy())

    a = images[0]
    vib = Vibrations(a, [4])
    vib.run()
    print(vib.get_frequencies())
    vib.summary()
    print(vib.get_mode(-1))
    vib.write_mode(-1, nimages=20)
