Accuracy of Rotational Parameters Predicted by High-Level Quantum-Chemical Calculations: Case Study of Sulfur-Containing Molecules of Astrochemical Interest

The accuracy of rotational parameters obtained from high-level quantum-chemical calculations is discussed for molecules containing second-row atoms. The main focus is on computed rotational constants for which two statistical analyses have been carried out. A first benchmark study concerns sulfur-bearing species and involves 15 molecules (for a total of 74 isotopologues). By comparing 15 different computational approaches, all of them based on the coupled-cluster singles and doubles approach (CCSD) augmented by a perturbative treatment of triple excitations, CCSD(T), we have analyzed the effects on computed rotational constants due to ( i) extrapolation to the complete basis-set limit, ( ii) correlation of core electrons, and ( iii) vibrational corrections to rotational constants. To extend the analysis to other molecules containing second-row elements, as well as to understand the effect of higher excitations, a second benchmark study involving 11 molecules (for a total of 54 isotopologues) has been performed. Finally, the rotational parameters of seven sulfur-containing molecules of astrochemical interest (CCS, C₃S, C₄S, C₅S, HCCS⁺, HC₄S⁺, and HOCS⁺/HSCO⁺) have been computed and compared to experimental data, when available, also addressing the direct comparison of simulated and experimental rotational spectra.