docs: 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