Determine the distance of space between
    each layer of atoms along c

def get_d(slab):
    """
    Determine the distance of space between
    each layer of atoms along c
    """
    sorted_sites = sorted(slab, key=lambda site: site.frac_coords[2])
    for i, site in enumerate(sorted_sites):
        if "%.6f" % (site.frac_coords[2]) == \
                        "%.6f" % (sorted_sites[i + 1].frac_coords[2]):
            continue
        else:
            d = abs(site.frac_coords[2] - \
                    sorted_sites[i + 1].frac_coords[2])
            break
    return slab.lattice.get_cartesian_coords([0, 0, d])[2]