Excitation operators and the extended random phase approximation: II. Applications

Abstract

The random phase approximation (RPA) and its extensions are tested by applying them to the π electron system of a series of linear polyenes in the PPP approximation, for which exact solutions are available. The importance of electron correlation is varied by altering the form of the electronic repulsion matrix elements in the PPP model. Emphasis is placed on a comparison between recently suggested refinements in the RPA and the more standard RPA scheme. The elements of the modifications of the RPA are illustrated by the simple example of ethylene. Calculations for longer polyenes (butadiene, hexatriene, and octatetraene) demonstrate that certain improvements in the RPA are essential if reliable results are to be obtained for highly correlated excited states. The results provide support for use of the extended RPA method in molecular excited state calculations.