code: def _Rzderiv(self,R,z,phi=0.,t=0.):
          
          Rz= R**2.+z**2.
          sqrtRz= numpy.sqrt(Rz)
          return -R*z*(-4.*Rz-3.*self.a*sqrtRz+3.*(self.a**2.+Rz+2.*self.a*sqrtRz)*numpy.log(1.+sqrtRz/self.a))*Rz**-2.5*(self.a+sqrtRz)**-2.
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)