Dual Nature of Chemical Bond and Vibration-Rotation Spectrum of the \(Be_2\) Molecule in the Ground \(X^1\)\(\Sigma\)\(^+_g\) State

Abstract

This review outlines the main results which lead to understanding the dual nature of the chemical bond in diatomic beryllium molecule in the ground \(X_1\)\(\Sigma\)\(^+_g\) state. It has been shown that the beryllium atoms are covalently bound at low-lying vibrational energy levels (\(\nu\) = 0 - 4), while at higher ones (\(\nu\) = 5 - 11) they are bound by van der Waals forces near the right turning points. High precision ab initio quantum mechanical calculations of Be2 resulted in the development of the modified expanded Morse oscillator potential function which contains all twelve vibrational energy levels [A. V. Mitin, Chem. Phys. Lett. 682, 30 (2017)]. The dual nature of chemical bond in Be₂ is evidenced as a sharp corner on the attractive branch of the ground state potential curve. Moreover, it has been found that the Douglas-Kroll-Hess relativistic corrections also show a sharp corner when presented in dependence on the internuclear separation. The difference in energy between the extrapolated and calculated multi-reference configuration interaction energies in dependence on the internuclear separation also exhibits singular point in the same region. The calculation of vibrational-rotational spectrum for the modified expanded Morse oscillator potential function and for function obtained with Slater-type orbitals [M. Lesiuk et al, Chem. Theory Comput. 15, 2470 (2019)] of the bound states of the beryllium dimer in the ground state was also considered in this review. Special attention was paid to the first calculations of the metastable vibrational-rotational complex-valued energy levels and the scattering length of the ground electronic state embedded in the continuum, along with the first theoretical estimations of the upper and lower border limits for the calculated vibrational-rotational energy levels of the bound as well as the metastable states. Such calculations are important for further experiments in laser spectroscopy of the beryllium dimer and for modeling its near-surface diffusion in connection with the well-known multifunctional use of beryllium alloys in innovative technologies of electronic, space and nuclear industries, including the ITER project.