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

Abstract

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, CS, CS, CS, HCCS, HCS, and HOCS/HSCO) have been computed and compared to experimental data, when available, also addressing the direct comparison of simulated and experimental rotational spectra.

A twist on the reaction of the CN radical with methylamine in the interstellar medium: new hints from a state-of-the-art quantum-chemical study

“…According to Figure 6, in addition to possible difficulties in producing these radicals inside the spectrometer cell, the assignment of their spectra can be complicated by the fact that the most intense transitions lie well in the submillimeterwave region. In fact, due to the propagation of the errors associated to the computed parameters when increasing the value of J, the uncertainties affecting the predicted transition values can be as large as 300-500 MHz (see, e.g., Alessandrini et al (2018)). However, one can rely on characteristic hyperfine pattern for helping the assignment procedure.…”

Extending the Applicability of the Semi-experimental Approach by Means of “Template Molecule” and “Linear Regression” Models on Top of DFT Computations

“… 2 , 5 , 73 , 74 An example is provided by the experimental investigation on the rotational spectrum of propargylimine, which also led to its detection in the interstellar medium. 54 In that work, the spectroscopic study was supported by quantum-chemical calculations, with equilibrium rotational constants obtained by means of the so-called “CCSD(T)/CBS + CV” composite scheme, 19 , 20 whose accuracy has been tested in refs ( 2 ) and ( 73 ). This composite scheme exploits the CCSD(T) method extrapolated to the complete basis set (CBS) limit (within the fc approximation) and incorporates the core-valence (CV) correlation correction.…”

Ab Initio Study of Fine and Hyperfine Interactions in Triplet POH

“…According to the literature on this topic (e.g., Refs. [ 14 , 21 , 65 ]), bond distances are accurate to 1–2 mÅ and angles to 0.3 . However, the true assessment of the accuracy of the computed geometries can only be addressed in comparison with experimental data.…”