def _z2deriv(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _z2deriv
        PURPOSE:
           evaluate the second vertical derivative for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the second vertical derivative
        HISTORY:
           2015-06-15 - Written - Bovy (IAS)
        """
        return (self._b2+R**2.-2.*z**2.)*(R**2.+z**2.+self._b2)**-2.5
NAME:
           _z2deriv
        PURPOSE:
           evaluate the second vertical derivative for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the second vertical derivative
        HISTORY:
           2015-06-15 - Written - Bovy (IAS)