code: def _zforce(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)
          dldz = jac[0,1]
          dndz = jac[1,1]
          return - (dldz * self._lderiv(l,n) + \
                    dndz * self._nderiv(l,n))
docs: NAME:
              _zforce
          PURPOSE:
              evaluate the vertical force for this potential
          INPUT:
              R - Galactocentric cylindrical radius
              z - vertical height
              phi - azimuth
              t - time
          OUTPUT:
              the vertical force
          HISTORY:
              2015-02-13 - Written - Trick (MPIA)