code: def _z2deriv(self,R,z,phi=0.,t=0.): #pragma: no cover
          
          raise AttributeError
          # Implementation above does not work bc SCF.z2deriv is not implemented
          r= numpy.sqrt(R**2.+z**2.)
          out= self._scf.z2deriv(R,z,phi=phi,use_physical=False)
          for a,s,ds,d2s,h,H,dH in zip(self._Sigma_amp,
                                     self._Sigma,self._dSigmadR,self._d2SigmadR2,
                                     self._hz,self._Hz,self._dHzdz):
              out+= 4.*numpy.pi*a*(H(z)/r**2.*(d2s(r)*z**2.+ds(r)*R**2./r)
                                   +2.*ds(r)*dH(z)*z/r+s(r)*h(z))
          return out
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:
             2016-12-26 - Written - Bovy (UofT/CCA)