docs: def collective_diffusion_coefficient( self ):
          """
          Returns the collective or "jump" diffusion coefficient, D_J.
  
          Args:
              None
  
          Returns:
              (Float): The collective diffusion coefficient, D_J.
          """
          if self.has_run:
              return self.atoms.collective_dr_squared() / ( 6.0 * self.lattice.time )
          else:
              return None