code: def _R2deriv(self,R,z,phi=0.,t=0.):
          
          return 1./(R**2.+(self._a+nu.sqrt(z**2.+self._b2))**2.)**1.5 \
              -3.*R**2./(R**2.+(self._a+nu.sqrt(z**2.+self._b2))**2.)**2.5
docs: NAME:
             _R2deriv
          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:
             2011-10-09 - Written - Bovy (IAS)