MICROCANONICAL ENSEMBLE MONTE-CARLO METHOD FOR DISCRETE-SYSTEMS

In an earlier work [Phys. Rev. A 44, 4061 (1991)] a method of carrying out Monte Carlo simulations in the microcanonical ensemble was discus sed and applied to systems described by continuous potentials. This me thod can also be used for discrete systems, e.g., spin, lattice gas, o r alloy type models where it furnishes a different way of exploring th e system than the canonical ensemble. A complete statistical mechanics and related thermodynamics exists for this microcanonical ensemble. W e give microcanonical ensemble fluctuation formulas for the specific h eat and constant energy susceptibility and relate these to the analogo us canonical ensemble expressions for the Ising model. As an example w e present simulation results for a two-dimensional Ising model and com pare the microcanonical and canonical ensemble calculation of various physical properties of the system. An interesting feature is the appea rance of a large (16%) ensemble difference between the specific heat i n canonical and microcanonical ensemble simulations for a 30 x 30 Isin g system in the vicinity of the maximum in the specific heat.