New problems for ab-initio valence bond theory

Abstract

The ab-initio valence bond method previously developed in our laboratory proved to be particularly useful in the description of electronic structure of molecules and the evaluation of ionization potentials. More recently, the photoelectron spectrum of pyridine has been described by the same method allowing for the assignment of the n+ and π+ bands. From these studies a variational valence bond method of calculating intermolecular forces stemmed, that was applied to the He-He and He-Li systems. Its main feature is the use of a nonorthogonal basis set in order to avoid over-extension basis error. Another application is the theoretical study of the formation of acetylene from two CH fragments which revealed a sudden change in spin coupling in the approach of the two fragments. A more elaborate procedure to obtain a basis set to be used in the description of excited states has been developed and successfully applied to the LiH molecule.