PMO theory

Abstract

This chapter provides a brief overview of the multiconfigurational approach in quantum chemistry. The method has been developed for studies of problems where a single configuration does not give a qualitatively correct description of the electronic structure. Examples of such cases are discussed in the chapter and it is illustrated how a valid wave function may be constructed. The concept of inactive and active orbitals is introduced and the complete active space wave function is defined. The choice of the active space is a crucial part of a compute dynamic correlation effects for multiconfigurational calculation. Without attempting to be complete, it has been illustrated that how this choice can be made for some typical electron structure problems. The problem of calculating the dynamic correlation energy is discussed with special emphasis on second order perturbation theory. Three-electron structure problems are discussed : ozone, the allyl radical, and the PbF molecule. © 2005 Elsevier B.V. All rights reserved.