def cn_occupation_energy( self, delta_occupation=None ):
        """
        The coordination-number dependent energy for this site.

        Args:
            delta_occupation (:obj:Dict(Str:Int), optional): A dictionary of a change in (site-type specific) coordination number, e.g. { 'A' : 1, 'B' : -1 }.
                If this is not None, the coordination-number dependent energy is calculated including these changes in neighbour-site occupations. Defaults to None

        Returns:
            (Float): The coordination-number dependent energy for this site.
        """
        nn_occupations = self.site_specific_nn_occupation()
        if delta_occupation:
            for site in delta_occupation:
                assert( site in nn_occupations )
                nn_occupations[ site ] += delta_occupation[ site ]
        return sum( [ self.cn_occupation_energies[ s ][ n ] for s, n in nn_occupations.items() ] )