code: def _z2deriv(self,R,z,phi=0.,t=0.):
          
          l,n    = bovy_coords.Rz_to_lambdanu    (R,z,ac=self._ac,Delta=self._Delta)
          jac    = bovy_coords.Rz_to_lambdanu_jac(R,z,            Delta=self._Delta)
          hess   = bovy_coords.Rz_to_lambdanu_hess(R,z,            Delta=self._Delta)
          dldz = jac[0,1]
          dndz = jac[1,1]
          d2ldz2 = hess[0,1,1]
          d2ndz2 = hess[1,1,1]
          return d2ldz2       * self._lderiv(l,n)  + \
                 d2ndz2       * self._nderiv(l,n)  + \
                 (dldz)**2    * self._l2deriv(l,n) + \
                 (dndz)**2    * self._n2deriv(l,n) + \
                 2.*dldz*dndz * self._lnderiv(l,n)
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:
              2015-02-13 - Written - Trick (MPIA)