code: def _R2deriv(self,R,z,phi=0.,t=0.):
          
          return self._denom(R, z)**-1.5 - 3.*R**2 * self._denom(R, z)**-2.5
docs: NAME:
              _Rforce
          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:
              2016-05-13 - Written - Aladdin