code: def _z2deriv(self,R,z,phi=0.,t=0.):
          
          return self._R2deriv(z,R)
docs: 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:
             2012-07-26 - Written - Bovy (IAS@MPIA)