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