code: def _Rzderiv(self,R,z,phi=0.,t=0.):
          
          return -3.*R*z*(R**2.+z**2.+self._b2)**-2.5
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:
             2015-06-15 - Written - Bovy (IAS)