code: def old_tracer_correlation( self ):
          
          if self.has_run:
              return self.atoms.sum_dr_squared() / float( self.number_of_jumps )
          else:
              return None
docs: 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`.