def z2deriv(self,R,Z,phi=0.,t=0.):
        """
        NAME:

           z2deriv

        PURPOSE:

           evaluate the second vertical derivative

        INPUT:

           R - Galactocentric radius (can be Quantity)

           Z - vertical height (can be Quantity)

           phi - Galactocentric azimuth (can be Quantity)

           t - time (can be Quantity)

        OUTPUT:

           d2phi/dz2

        HISTORY:

           2012-07-25 - Written - Bovy (IAS@MPIA)

        """
        try:
            return self._amp*self._z2deriv(R,Z,phi=phi,t=t)
        except AttributeError: #pragma: no cover
            raise PotentialError("'_z2deriv' function not implemented for this potential")
NAME:

           z2deriv

        PURPOSE:

           evaluate the second vertical derivative

        INPUT:

           R - Galactocentric radius (can be Quantity)

           Z - vertical height (can be Quantity)

           phi - Galactocentric azimuth (can be Quantity)

           t - time (can be Quantity)

        OUTPUT:

           d2phi/dz2

        HISTORY:

           2012-07-25 - Written - Bovy (IAS@MPIA)