code: def k(self):
          
          try:
              return self._k
          except AttributeError:
              self._k = _np.sqrt(_np.pi/8) * e**2 * self.nb0 * self.sig_y / ( e0 * self.m * c**2)
              return self._k
docs: Driving force term: :math:`r'' = -k \\left( \\frac{1-e^{-r^2/2{\\sigma_r}^2}}{r} \\right)`