DIFFUSION REORDERING KINETICS IN LATTICE-GAS SYSTEMS - TIME EVOLUTIONOF CONFIGURATIONAL ENTROPY AND INTERNAL ENERGY

The reordering kinetics of a diffusion lattice-gas system of adsorbate s with nearest- and next-nearest-neighbor interactions on a square lat tice is studied within a dynamic Monte Carlo simulation, as it evolves towards the equilibrium from a given initial configuration, at a cons tant temperature. The diffusion kinetics proceeds through adsorbate ho ppings to empty nearest-neighboring sites (Kawasaki dynamics). The Mon te Carlo procedure allows a ''real'' time definition from the local tr ansition rates, and the configurational entropy and internal energy ca n be obtained from the lattice configuration at a.ny instant t by coun ting the local clusters and using the C-2 approximation of the cluster variation method. These state functions are then used in their nonequ ilibrium form as a direct measure of reordering along the time. Differ ent reordering processes are analyzed within this approach, presenting a rich variety of behaviors. It can also be shown that the time deriv ative of entropy (times temperature) is always equal to or lower than the time derivative of energy, and that the reordering path is always strongly dependent on the initial order, presenting in some cases an ' 'invariance'' of the entropy function to the magnitude of the interact ions as far as the final order is unaltered.