def _Rzderiv(self,R,z,phi=0.,t=0.):
        """
        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:
            2015-02-13 - Written - Trick (MPIA)
        """
        l,n    = bovy_coords.Rz_to_lambdanu    (R,z,ac=self._ac,Delta=self._Delta)
        jac    = bovy_coords.Rz_to_lambdanu_jac(R,z,            Delta=self._Delta)
        hess   = bovy_coords.Rz_to_lambdanu_hess(R,z,            Delta=self._Delta)
        dldR = jac[0,0]
        dndR = jac[1,0]
        dldz = jac[0,1]
        dndz = jac[1,1]
        d2ldRdz = hess[0,0,1]
        d2ndRdz = hess[1,0,1]
        return d2ldRdz              * self._lderiv(l,n)  + \
               d2ndRdz              * self._nderiv(l,n)  + \
               dldR*dldz            * self._l2deriv(l,n) + \
               dndR*dndz            * self._n2deriv(l,n) + \
               (dldR*dndz+dldz*dndR)* self._lnderiv(l,n)
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:
            2015-02-13 - Written - Trick (MPIA)