Abstract:

In this study, we used the multireference complete active space perturbation theory (CASPT2) method to calculate the equilibrium nuclear distances and wave functions of the 2nd-row homonuclear diatomic molecules in the ground-state. The wave functions of Li₂, N₂, O₂, and F₂ molecules are dominated by a single electron configuration; thus, their bonding properties can be analyzed on the base of a single configuration. In contrast, Be₂, B₂, and C₂ molecules have obvious multiconfigurational characteristics, so considering the contributions of multiple configurations is necessary to obtain consistent results with the experimental observations. By introducing the contribution of the second most important configuration in the calculation of bond order or using the fractional electron occupation number of natural orbitals, we have rationalized the existence of the Be₂ molecule and its non-zero Be―Be bond order. Additionally, we determined that the bond orders in B₂ and C₂ are 1–2 and 2–3, respectively. The results obtained here can be used as a supplement to the simple molecular orbital theory to help students understand the structure and bonding nature of diatomic molecules. The present work emphasizes the necessity of multiconfiguration characteristics of wave functions to understand molecular structure, and verifies the feasibility of introducing relatively simple, cost effective, multireference calculations to assist in teaching.

Key words: Homonuclear diatomic molecules, Molecular orbital theory, Multireference method, Multiconfigurational wave function, Bond order