Spectroscopic Properties of the Barium Sulfide Molecule in Its Low-Lying Electronic States X1Σ+, a3Π, A'1Π, b3Σ+, and A1Σ+

Abstract

The spectroscopic constants re, ωe, ωexe, αe, De, βe and atomization energy of the barium sulfide molecule, BaS, in its electronic ground state X1Σ+ are calculated using the coupled cluster singles, doubles, triples, and quadruples method CCSDTQ in conjunction with complete basis set (CBS) limit extrapolation and taking spin-orbit coupling into account. Excellent agreement of the calculation results with the available experimental data is observed, e.g., the theoretical values of re and ωe deviate from those obtained from spectroscopic measurements by less than 0.001 A and 1 cm–1, respectively. Correlation of the barium core 5s and 5p electrons is shown to contribute substantially to the values of molecular parameters. Based on an approach combining the coupled cluster theory at the CCSD(T)/CBS level with the multireference configuration interaction (MRCISD+Q) method, a high-accuracy theoretical description of the low-lying excited triplet (a3Π, b3Σ+) and singlet (A′1Π, A1Σ+) electronic states of the BaS molecule is reported for the first time.