A Brief Tour Through Some Valence Bond Outputs and Terminology

Chapter 2 displays and comments on outputs of modern ab initio VB calculations using initially a minimal basis set and a double-zeta one, in the TURTLE and XMVB packages. The H 2 molecule displays one major covalent structure and two less important degenerate ionic structures. The output shows the coefficients of the VB structures and their weights, the Hamiltonian matrix, and the matrix of overlaps between the VB structures. The resonance energy due to the mixing of the covalent and ionic structures makes a small contribution to the bonding energy. The VB mixing diagram shows how to qualitatively construct the ground state and excited states using perturbation theory and experimental data. The covalent structure is the lowest one also in the HF molecule, but the dominant ionic structure is close in energy. This covalent–ionic proximity leads to a very large covalent–ionic resonance energy.