Semi-empirical calculation on core ionizations and simultaneous excitations by use of the π-electron and equivalent core approximations. The theoretical foundations of the method and the application to some π-conjugated molecules

Abstract

The theoretical foundations of the semi-empirical calculation with the equivalent core approximation are discussed. The equivalency of the core- hole-localized model with the core-hole-delocalized model and the importance of including doubly-excited configurations in describing the core ionization and the simultaneous excitation (the shake-up transition) are shown from the discussion on a simplified diatomic model system. Numerical calculations with the equivalent core approximation are performed on ethylene, benzene and butadience by use of the full CI semi-empirical method within the framework of the π - electron approximation.