伊辛模型

\begin{equation} \left\langle M \right\rangle = \frac{\sum_i M_i \mathrm{e} ^{-M_i H \beta}}{\sum_i M_i \mathrm{e} ^{-M_i H\beta}} = -kT \frac{\partial}{\partial{H}} \ln Q \end{equation}

\begin{equation} U = - \left\langle M \right\rangle H \end{equation}