Chapter 3. Theoretical organic chemistry

Abstract

The high level of activity in the area of density functional theory continues and a number of useful assessments of various aspects have appeared. Notable amongst these is a detailed review of the methodology of the DGauss program' and its application to many organic systems. Fan and Ziegler2 also provide an assessment of their density functional formalism, through its application to the calculation of activation energies for elementary organic reactions, the results of which are very encouraging. Application of density functional theory to the calculation of 27 homolytic dissociation reactions using generalized valence bond (GVB) reference wavefunctions3 and a modest basis set (&3 lG(D)), show an average error of 4 kcal: mol-'. The use of the same basis set and Hartree-Fock reference wavefunctions with a density function treatment of exchange and correlation4 for a set of 106 experimentally well established atomization energies, ionization potentials, and proton affinities show a mean absolute deviation of only 4.18 kcal mol-'. Other tests of density functional using the large G1 database of Pople and co-workers' have also shown these approaches to provide accurate and computationally efficient methods for studying chemical reactions.Though not directly related to density functional theory, an interesting analysis of the effect of electron correlation on charge density distributions in molecules has been carried out for the widely used MP2, CISD, and QCISD wavefunctions.8 Compared to the QCI density, MP2 is found to consistently exaggerate the loss in charge density in bonding regions leading to an increase in bond length, whereas CISD is found to underestimate the shift.Pople and co-workers have provided a detailed overview of their approach to the molecular orbital theory ofexcited states.' The method is now applicable to quite large