Gets the structure of single atom adsorption.
        slab structure from the Slab class(in [0, 0, 1])

        Args:
            indices ([int]): Indices of sites on which to put the absorbate.
                Absorbed atom will be displaced relative to the center of
                these sites.
            specie (Specie/Element/str): adsorbed atom species
            distance (float): between centers of the adsorbed atom and the
                given site in Angstroms.

def add_adsorbate_atom(self, indices, specie, distance):
        """
        Gets the structure of single atom adsorption.
        slab structure from the Slab class(in [0, 0, 1])

        Args:
            indices ([int]): Indices of sites on which to put the absorbate.
                Absorbed atom will be displaced relative to the center of
                these sites.
            specie (Specie/Element/str): adsorbed atom species
            distance (float): between centers of the adsorbed atom and the
                given site in Angstroms.
        """
        # Let's do the work in cartesian coords
        center = np.sum([self[i].coords for i in indices], axis=0) / len(
            indices)

        coords = center + self.normal * distance / np.linalg.norm(self.normal)

        self.append(specie, coords, coords_are_cartesian=True)