Returns the surface area of the adsorbed system per
        unit area of the primitive slab system.

def get_unit_primitive_area(self):
        """
        Returns the surface area of the adsorbed system per
        unit area of the primitive slab system.
        """

        A_ads = self.surface_area
        A_clean = self.clean_entry.surface_area
        n = (A_ads / A_clean)
        return n