docs: 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