Weiss平均值理论: 近似认为相邻自旋间的作用由一个总的平均场<σ>代替, 所有自旋独立地存在于这个平均场中, 所有自旋的场能即为体系总能量。-Weiss average theory: approximation that the role between the adjacent spin by a general mean-field < σ>　to replace all the spin-independent existence in this mean-field, all spin field energy system shall be the total energy.