Returns the primitive unit surface area density of the
            adsorbate.

def get_monolayer(self):
        """
        Returns the primitive unit surface area density of the
            adsorbate.
        """

        unit_a = self.get_unit_primitive_area
        Nsurfs = self.Nsurfs_ads_in_slab
        Nads = self.Nads_in_slab
        return Nads / (unit_a * Nsurfs)