def _Rforce(self,R,z,phi=0.,t=0.):
        """
        NAME:
            _Rforce
        PURPOSE:
            evaluate the radial force for this potential
        INPUT:
            R - Galactocentric cylindrical radius
            z - vertical height
            phi - azimuth
            t - time
        OUTPUT:
            the radial force = -dphi/dR
        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)
        dldR = jac[0,0]
        dndR = jac[1,0]
        return - (dldR * self._lderiv(l,n) + \
                  dndR * self._nderiv(l,n))
NAME:
            _Rforce
        PURPOSE:
            evaluate the radial force for this potential
        INPUT:
            R - Galactocentric cylindrical radius
            z - vertical height
            phi - azimuth
            t - time
        OUTPUT:
            the radial force = -dphi/dR
        HISTORY:
            2015-02-13 - Written - Trick (MPIA)