Returns the collective correlation factor, f_I

        Args:
            None

        Returns:
            (Float): The collective correlation factor, f_I.

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

def old_collective_correlation( self ):
        """
        Returns the collective correlation factor, f_I

        Args:
            None

        Returns:
            (Float): The collective correlation factor, f_I.

        Notes:
            This function assumes that the jump distance between sites has
            been normalised to a=1. If the jumps distance is not equal to 1
            then the value returned by this function should be divided by a^2.
            Even better, use self.collective_correlation
        """
        if self.has_run:
            return self.atoms.collective_dr_squared() / float( self.number_of_jumps )
        else:
            return None