Chapter 4 Bond Breaking in Quantum Chemistry

Abstract

This chapter describes the different aspects of bond breaking in quantum chemistry. Ab initio quantum chemical methods can predict the equilibrium properties of small molecules in the gas phase to near-spectroscopic accuracy. A detailed understanding of a chemical reaction often requires the knowledge of its dynamics, which in turn requires the knowledge of the potential energy surface. By allowing the alpha and beta electrons to occupy different orbitals, the wave function can avoid the unphysical ionic terms. This approach is called unrestricted “Hartree–Fock (UHF),” as opposed to the usual, restricted Hartree–Fock (RHF) method. The UHF potential energy curve is qualitatively correct, but often quantitatively poor. It also has the rather undesirable property that it is not an Eigen function of the electronic spin operator. CASSCF is the most widely used quantum chemical method for bond-breaking reactions. CASSCF wave functions are usually easier to converge than general multiconfigurational self-consistent-field (MCSCF) wave functions and the ambiguity of selecting individual configurations is removed. The problematic ionic terms that make the RHF energy too high for large separations can also be avoided by using nonorthogonal orbitals in the valence bond approach.