code: def _Rzderiv(self,R,z,phi=0.,t=0.):
          
          if not self.isNonAxi:
              phi= 0.
          x,y,z= bovy_coords.cyl_to_rect(R,phi,z)
          if not self._aligned:
              raise NotImplementedError("2nd potential derivatives of TwoPowerTriaxialPotential not implemented for rotated coordinated frames (non-trivial zvec and pa)")
          phixz= self._2ndderiv_xyz(x,y,z,0,2)
          phiyz= self._2ndderiv_xyz(x,y,z,1,2)
          return numpy.cos(phi)*phixz+numpy.sin(phi)*phiyz
docs: NAME:
             _Rzderiv
          PURPOSE:
             evaluate the mixed radial, vertical derivative for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the mixed radial, vertical derivative
          HISTORY:
             2016-06-15 - Written - Bovy (UofT)