code: def _Rzderiv(self,R,z,phi=0.,t=0.):
          
          if not self.isNonAxi:
              phi= 0.
          x,y,z= self._compute_xyz(R,phi,z,t)
          phixza= self._2ndderiv_xyz(x,y,z,0,2)
          phiyza= self._2ndderiv_xyz(x,y,z,1,2)
          ang = self._omegab*t + self._pa
          c, s = np.cos(ang), np.sin(ang)
          phixz = c*phixza + s*phiyza
          phiyz = -s*phixza + c*phiyza
          return np.cos(phi)*phixz + np.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