code: def _Rzderiv(self,R,z,phi=0.,t=0.): #pragma: no cover
          
          raise AttributeError
          # Implementation above does not work bc SCF.Rzderiv is not implemented
          r= numpy.sqrt(R**2.+z**2.)
          out= self._scf.Rzderiv(R,z,phi=phi,use_physical=False)
          for a,ds,d2s,H,dH in zip(self._Sigma_amp,self._dsigmadR,
                                   self._d2SigmadR2,self._Hz,self._dHzdz):
              out+= 4.*numpy.pi*a*(H(z)*R*z/r**2.*(d2s(r)-ds(r)/r)
                                   +ds(r)*dH(z)*R/r)
          return out
docs: NAME:
             _Rzderiv
          PURPOSE:
             evaluate the mixed R,z derivative for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             d2phi/dR/dz
          HISTORY:
             2016-12-26 - Written - Bovy (UofT/CCA)