def old_tracer_correlation( self ):
        """
        Deprecated tracer correlation factor for this simulation.
       
        Args:
            None

        Returns:
            (Float): The tracer correlation factor, f.

        Notes:
            This function assumes that the jump distance between sites has
            been normalised to a=1. If the jump distance is not equal to 1
            then the value returned by this function should be divided by a^2.
            Even better, use `self.tracer_correlation`.
        """
        if self.has_run:
            return self.atoms.sum_dr_squared() / float( self.number_of_jumps )
        else:
            return None
Deprecated tracer correlation factor for this simulation.
       
        Args:
            None

        Returns:
            (Float): The tracer correlation factor, f.

        Notes:
            This function assumes that the jump distance between sites has
            been normalised to a=1. If the jump distance is not equal to 1
            then the value returned by this function should be divided by a^2.
            Even better, use `self.tracer_correlation`.