def _R2deriv(self,R,z,phi=0.,t=0.):
        """
        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 
        """
        return self._denom(R, z)**-1.5 - 3.*R**2 * self._denom(R, z)**-2.5
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
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