4 - Approximate Molecular Orbital Theories

Abstract

Starting from the Hartree–Fock theory developed in Chapter 3, relationship has been developed between the approximate quantum chemical methods based on zero-differential overlap approximation and the ab initio methods. While the focus in this chapter is on some of the more popular methods developed in the research groups of Pople, such as complete neglect of differential overlap, intermediate neglect of differential overlap (INDO), and neglect of diatomic differential overlap methods, and Dewar like modified INDO, modified neglect of diatomic overlap, Austin Model-1, and Parameterization Method 3, etc., methods like INDO/S developed by Ridley and Zener for spectral studies and CS-INDO developed by Momicchioli for conformational and spectral studies have also received due attention. The chapter includes semiempirical methods for planar-conjugated systems such as Hückel theory and the Pariser–Parr–Pople method, in which electron interactions are taken into account. Extended Hückel theory, which is an improvement over the Hückel theory and takes into account all the valence electrons in the molecule, is also included. The practical aspects related to the different semiempirical methods, have been illustrated with examples and successes and failures of the methods examined.