code: def _Rforce(self,R,z,phi=0.,t=0.):
          
          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))
docs: 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)