 Statistical Hamiltonian

A statistical Hamiltonian assigns an energy to a microscopic configuration. Its Boltzmann weight defines the canonical ensemble, and summing or integrating that weight gives a partition function. A reduced Hamiltonian absorbs inverse temperature into its coefficients, so the weight is $e^{-H_{\mathrm{reduced}}}$. An Ising model uses discrete Ising spin configurations, while Landau-Ginzburg theory uses a coarse scalar field.