def METHOD_NAME(A='Li', B='Cl'):
    assert A in ['Li', 'Mg']
    # Add Na, K
    assert B in ['H', 'F', 'Cl', 'O']
    # Add Br, I
    from ase.lattice import bulk
    if A=='Li':
        if B=='H':
            ase_atom = bulk('LiH', 'rocksalt', a=4.0834*A2B)
        elif B=='F':
            ase_atom = bulk('LiF', 'rocksalt', a=4.0351*A2B)
        elif B=='Cl':
            ase_atom = bulk('LiCl', 'rocksalt', a=5.13*A2B)
    elif A=='Mg' and B=='O':
        ase_atom = bulk('MgO', 'rocksalt', a=4.213*A2B)
    else:
        raise NotImplementedError('No formula found for system %s %s. '
                                  'Choose a different system?  Or add it to the list!' % (A, B))
    return ase_atom