def _R2deriv(self,R,z,phi=0.,t=0.):
        """
        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)
        """
        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