Direct Calculation of Absolute Free Energy for Lattice Systems by Monte Carlo Sampling of Finite-Size Dependence

It is shown that the absolute free energy of lattice spin systems can be calculated directly by a novel application of Monte Carlo sampling of finite-size dependence. Results are obtained for the two- and three-dimensional Ising models at $T_c$ and are consistent with recent finite-size scaling-theory predictions. The free energy is in excellent agreement with Onsager's exact solution for two dimensions and with series expansions for three dimensions. The $U_0$ is measured for the first time. Results are consistent with the proposed universality.