Check if we have same number of equivalent sites on both surfaces.
        This is an alternative to checking Laue symmetry (is_symmetric())
        if we want to ensure both surfaces in the slab are the same

def have_equivalent_surfaces(self):
        """
        Check if we have same number of equivalent sites on both surfaces.
        This is an alternative to checking Laue symmetry (is_symmetric())
        if we want to ensure both surfaces in the slab are the same
        """

        # tag the sites as either surface sites or not
        surf_sites_dict = self.get_surface_sites(tag=True)

        a = SpacegroupAnalyzer(self)
        symm_structure = a.get_symmetrized_structure()

        # ensure each site on one surface has a
        # corresponding equivalent site on the other
        equal_surf_sites = []
        for equ in symm_structure.equivalent_sites:
            # Top and bottom are arbitrary, we will just determine
            # if one site is on one side of the slab or the other
            top, bottom = 0, 0
            for s in equ:
                if s.is_surf_site:
                    if s.frac_coords[2] > self.center_of_mass[2]:
                        top += 1
                    else:
                        bottom += 1
            # Check to see if the number of equivalent sites
            # on one side of the slab are equal to the other
            equal_surf_sites.append(top == bottom)

        return all(equal_surf_sites)