def k(self):
        """
        Driving force term: :math:`r'' = -k \\left( \\frac{1-e^{-r^2/2{\\sigma_r}^2}}{r} \\right)`
        """
        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
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
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
Driving force term: :math:`r'' = -k \\left( \\frac{1-e^{-r^2/2{\\sigma_r}^2}}{r} \\right)`
Driving force term : : math : r = - k \\ left ( \\ frac { 1 - e^ { - r^2 / 2 { \\ sigma_r } ^2 }} { r } \\ right )