ABSTRACT

A well-known approximate method of quantum chemistry is the Rayleigh-Schrödinger perturbation theory. The task of the present paper is to survey this method from the point of view of its dependence on the time parameter. Two alternative pathways of the time events caused by the perturbation effect exerted on a non-degenerate quantum state are examined. The first way – which is traditional and applies the Feynman diagrams – assumes a straight-linear sequence of events along the time scale, the second way – which is less known and is the main subject of the present review – arranges a similar sequence of events along a circular scale of time.

The efficiency of the both kinds of the time scale can be checked in practical perturbation calculations. This comparison of two scales shows a predominant advantage of the circular time scale over the straight-linear scale of time.