Dynamical electron correlation and the chemical bond. III. Covalent bonds in the A2 molecules (A = C–F)

For most molecules the spin-coupled generalized valence bond (SCGVB) wavefunction accounts for the effects of non-dynamical electron correlation. The remaining errors in the prediction of molecular properties and the outcomes of molecular processes are then solely due to dynamical electron correlation. In this article we extend our previous studies of the effects of dynamical electron correlation on the potential energy curves and spectroscopic constants of the AH and AF (A = B–F) molecules to the homonuclear diatomic molecules, A2 (A = C–F). At large R the magnitude of ΔEDEC(R), the correlation energy of the molecule relative to that in the atoms, increases nearly exponentially with decreasing R, just as we found in the AH and AF molecules. But, as R continues to decrease the rate of increase in the magnitude of ΔEDEC(R) slows, eventually leading to a minimum for C2–O2. Examination of the SCGVB wavefunction for the N2 molecule around the minimum in ΔEDEC(R) did not reveal a clear cause for this puzzling behavior. As before, the changes in ΔEDEC(R) around Re were found to have an uneven effect on the spectroscopic constants of the A2 molecules.