Calculates the dipole of the Slab in the direction of the surface
        normal. Note that the Slab must be oxidation state-decorated for this
        to work properly. Otherwise, the Slab will always have a dipole of 0.

def dipole(self):
        """
        Calculates the dipole of the Slab in the direction of the surface
        normal. Note that the Slab must be oxidation state-decorated for this
        to work properly. Otherwise, the Slab will always have a dipole of 0.
        """
        dipole = np.zeros(3)
        mid_pt = np.sum(self.cart_coords, axis=0) / len(self)
        normal = self.normal
        for site in self:
            charge = sum([getattr(sp, "oxi_state", 0) * amt
                          for sp, amt in site.species.items()])
            dipole += charge * np.dot(site.coords - mid_pt, normal) * normal
        return dipole