A Computational Study of the Vibrational and Rotational -Factors of the Diatomic Molecules LiH, LiF, CO, CS, SiO, and SiS.

The purpose of this article is to present theoretical values for the vibrational and rotational -factors of several diatomic molecules. The calculations were carried out at the multiconfigurational self-consistent field (MCSCF) level of theory. To determine the most reliable method and basis set for these calculations, the Hartree-Fock (HF) and density functional theory (DFT) approaches were also considered. Different DFT functionals, including B3LYP, BHandHLYP, PBE0, B3PW91, and KT3, have been employed. Furthermore, different active spaces were evaluated to optimize MCSCF. To establish the accuracy of the methods, the computed rotational -factors were compared to experimental values. The benchmark study of CO and CS shows that the MCSCF method provides the most reliable results and that the aug-cc-pCV5Z basis set is the most sufficient. The aug-cc-pCVQZ basis set for Li and aug-cc-pV5Z basis set for H gave the best results for LiH. The active spaces tested for CO and CS do not yet converge toward the experimental values when more determinants were included. However, if the -factors are vibrationally averaged, the computed values are seen to move toward the experimental value.