Absolute entropy and free energy of fluids using the hypothetical scanning method.I.Calculation of transition probabilities from local grand canonical partition functions

摘要

The hypothetical scanning (HS)method provides the absolute entropy and free energy from a Boltzmann sample generated by Monte Carlo,molecular dynamics or any other exact simulation procedure.Thus far HS has been applied successfully to magnetic and pol;ymer chain models;in this paper and the following one it is extended to fluid systems by treating a lennard-jones model of argon.With HS a probability P_i approximating the Boltzmann probability of system configuration i is calculated with a stepwise reconstruction procedured,based on adding atoms gradually layer-by-layer to an initially empty voume,where theky are replaced in their positions at i.At each step a transition probability (TP) is obtained from local grand canonical partition functions calculated over a limited space of the still unvisited (future)volume,tjhe larger this space the better the approximation.P i is the product of the step TPs,where In P_i is an upper bound of the absolute entropy,,which leads to upper and lover bounds for the free energy.We demonstrate that very good results for the entropy and the free energy can be obtained for a wide range of densities of the argon system by calculation TPs that are based on only a very limited future volume.