Nearest-neighbour interaction contribution to the change in system energy if this jump were accepted.

        Args:
            None

        Returns:
            (Float): delta E (nearest-neighbour)

def nearest_neighbour_delta_E( self ):
        """
        Nearest-neighbour interaction contribution to the change in system energy if this jump were accepted.

        Args:
            None

        Returns:
            (Float): delta E (nearest-neighbour)
        """
        delta_nn = self.final_site.nn_occupation() - self.initial_site.nn_occupation() - 1 # -1 because the hopping ion is not counted in the final site occupation number
        return ( delta_nn * self.nearest_neighbour_energy )