Relaxation Kinetics of the Path Probability Method

The fundamentals of the path probability method (PPM) devised by Kikuchi are reviewed by using a spin flip model and putting a stress upon the logical structures of the PPM. After the PPM is formulated taking a system itself as the basic cluster for an ensemble of equivalent systems, the derivation of an approximation series is discussed. Not only at each approxima­ tion level but also in the general formulation, the master equation and the path integral representation of time evolution of the system are derived on the basis of the Markovian property and the variational principle inherent in the PPM. It is also shown that the anti-causal path against the natural path leading to the thermal equilibrium brings about the increase in the free energy. As a concrete example of the general formulation the pair approximation is treated in detail.