def tracer_diffusion_coefficient( self ):
        """
        Tracer diffusion coefficient, D*.

        Args:
            None

        Returns:
            (Float): The tracer diffusion coefficient, D*.
        """
        if self.has_run:
            return self.atoms.sum_dr_squared() / ( 6.0 * float( self.number_of_atoms ) * self.lattice.time )
        else:
            return None