Returns the collective or "jump" diffusion coefficient, D_J.

        Args:
            None

        Returns:
            (Float): The collective diffusion coefficient, D_J.

def collective_diffusion_coefficient( self ):
        """
        Returns the collective or "jump" diffusion coefficient, D_J.

        Args:
            None

        Returns:
            (Float): The collective diffusion coefficient, D_J.
        """
        if self.has_run:
            return self.atoms.collective_dr_squared() / ( 6.0 * self.lattice.time )
        else:
            return None