code: def _R2deriv(self,R,z,phi=0.,t=0.):
          
          r= nu.sqrt(R*R+z*z)
          return 4.*nu.pi*r**(-2.-self.alpha)*nu.exp(-(r/self.rc)**2.)*R**2.\
              +self._mass(r)/r**5.*(z**2.-2.*R**2.)
docs: NAME:
             _Rderiv
          PURPOSE:
             evaluate the second radial derivative for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the second radial derivative
          HISTORY:
             2013-06-28 - Written - Bovy (IAS)