def test_atoms_formula():
    from ase.build import fcc111, add_adsorbate
    import warnings

    # some random system
    slab = fcc111('Al', size=(2, 2, 3))
    add_adsorbate(slab, 'C', 2.5, 'bridge')
    add_adsorbate(slab, 'C', 3.5, 'bridge')
    add_adsorbate(slab, 'H', 1.5, 'ontop')
    add_adsorbate(slab, 'H', 1.5, 'fcc')
    add_adsorbate(slab, 'C', 0.5, 'bridge')
    add_adsorbate(slab, 'C', 1.5, 'bridge')

    assert slab.get_chemical_formula(mode='hill') == 'C4H2Al12'
    assert slab.get_chemical_formula(mode='metal') == 'Al12C4H2'
    all_str = 'Al' * 12 + 'C' * 2 + 'H' * 2 + 'C' * 2
    assert slab.get_chemical_formula(mode='all') == all_str
    reduce_str = 'Al12C2H2C2'
    assert slab.get_chemical_formula(mode='reduce') == reduce_str

    assert slab.get_chemical_formula(mode='hill', empirical=True) == 'C2HAl6'
    assert slab.get_chemical_formula(mode='metal', empirical=True) == 'Al6C2H'


    # check for warning if empirical formula is not available
    for mode in ('all', 'reduce'):
        with warnings.catch_warnings(record=True) as w:
            # Cause all warnings to always be triggered.
            warnings.simplefilter('always')
            # Trigger a warning.
            slab.get_chemical_formula(mode=mode, empirical=True)
            # Verify some things
            assert len(w) == 1
            assert issubclass(w[-1].category, Warning)