Multiconfigurational Study on the Contribution of the Nondynamical and Dynamical Correlation Energies to the Dissociation Energies of Li₂-to-F₂ Molecules

Approaching the exact solution of nonrelativistic electronic wave equation in molecules and the calculation of thermochemical quantities with high accuracy, without the help of extrapolation techniques or complicating r₁₂ terms, is still a challenging task in quantum chemistry. Recent advances in computer hardware made it possible to achieve very high accuracy. However, it is still difficult to describe many chemically important situations where the key problem is the inherent multideterminental nature of the wave function. The inclusion of nondynamical correlation in terms of internal correlation, semi-internal correlation, and orbital polarization effects is crucial in multireference systems. The multireference character of the system at large separations, including the dissociation region, is known to be the major difficulty for the state-of-the-art theoretical calculations. Accurate estimation of PECs is contingent upon the proper treatment of the intricate interplay of nondynamical and dynamical correlation over the dissociation path. Despite their simple bonding schemes, the homonuclear diatomic molecules of the first row atoms are still notoriously difficult to describe from first-principles due to their varied electronic structures. In this work, the contribution of nondynamical and dynamical correlations to dissociation energies of diatomic molecules of first row atoms (Li₂ to F₂) was investigated in great detail. Starting from a large active space, the multiconfigurational type NCMET(ND) wave function was calculated which includes manually chosen nondynamical correlation terms. The def2-QZVP and correlation-consistent quadruple-ζ basis sets (aug-cc-pVQZ) have been opted in all calculations. A complete basis set (CBS) limit has also been calculated through 5Z and 6Z basis sets. Internal and semi-internal type triple, quadruple, and higher excitations were also included. The contributions of scalar relativistic effects along with the spin-orbit interaction to dissociation energy were taken into account. In addition, CASCI, CASVB, SOCI, Mk-MRCC, and NCMET (nonclosed shell many-electron theory) type calculations were performed to cover the effects of dynamical correlation. The core correlation effects including the core polarization phenomena and the corrections beyond adiabatic approximation were also considered. The results were also compared to full CI computations which are also performed in this work. The expectation values of various one-electron properties were also tested for NCMET(ND) and SOCI methods.