Dynamical Electron Correlation and the Chemical Bond. IV. Covalent Bonds in A₂ Molecules (A = N-As and F-Br)

In a series of recent papers, we investigated the effect of dynamical electron correlation on the potential energy curves and spectroscopic constants of several diatomic molecules, including the simple diatomic hydrides (AH) and the more complex diatomic fluorides (AF) and homonuclear diatomic molecules (A₂) with A = B-F (AF) or A = C-F (A₂), respectively. Our goal was to understand the dependence of the dynamical electron correlation energy, E_DEC, on the internuclear distance, R, and quantify how dynamical electron correlation influences the spectroscopic constants (D_e, R_e, and ω_e) of these molecules. At large R, we found that the magnitude of E_DEC(R) had a simple dependence on R, with E_DEC(R) increasing nearly exponentially with decreasing R. However, as R continued to decrease, there were significant variations in E_DEC(R). These variations led to differing changes in the predicted spectroscopic constants of the molecules. In many molecules, the changes in E_DEC(R) could be correlated with changes in the underlying spin-coupled generalized valence bond wave function, either in the orbitals or the spin-coupling coefficients. In the current paper, we extend these studies to higher main group elements, comparing the effects of E_DEC(R) on P₂ and As₂ versus N₂, and on Cl₂ and Br₂ versus F₂. We find that there are significant differences between the effects of dynamical electron correlation on the molecules in the first and subsequent rows of the periodic table.