code: def _Rzderiv(self,R,z,phi=0.,t=0.):
          
          sqrtbz= nu.sqrt(self._b2+z**2.)
          asqrtbz= self._a+sqrtbz
          if isinstance(R,float) and sqrtbz == asqrtbz:
              return -(3.*R*z/(R**2.+asqrtbz**2.)**2.5)
          else:
              return -(3.*R*z*asqrtbz
                       /sqrtbz/(R**2.+asqrtbz**2.)**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:
             2013-08-28 - Written - Bovy (IAS)