def Nsurfs_ads_in_slab(self):
        """
        Returns the TOTAL number of adsorbed surfaces in the slab
        """

        struct = self.structure
        weights = [s.species.weight for s in struct]
        center_of_mass = np.average(struct.frac_coords,
                                    weights=weights, axis=0)

        Nsurfs = 0
        # Are there adsorbates on top surface?
        if any([site.species_string in self.ads_entries_dict.keys() for \
                site in struct if site.frac_coords[2] > center_of_mass[2]]):
            Nsurfs += 1
        # Are there adsorbates on bottom surface?
        if any([site.species_string in self.ads_entries_dict.keys() for \
                site in struct if site.frac_coords[2] < center_of_mass[2]]):
            Nsurfs += 1

        return Nsurfs
Returns the TOTAL number of adsorbed surfaces in the slab