Explicitly correlated and CBS study of the internal rotation in CH(3)SH, CH(3)SD, CD(3)SH, CD(3)SD, (13)CH(3)SH, and CH(3)(34)SH molecules, including corrections on anharmonic ZPVE and relativistic effects.

The energies of stationary torsional states of methanethiol (methyl mercaptan; CH₃SH) and several of its isotopic analogs (CH₃SD, CD₃SH, CD₃SD, CH₃SH, and CH₃SH), which are of considerable interest in astronomical studies, were calculated using several levels of theory, including extrapolation to the complete basis set (CBS) limit and explicitly correlated methods. The potential energy function and kinetic coefficients were computed at the CCSD(T)-F12-RI/Aug-cc-pVTZ-F12, MP2/CBS(Aug-ano-VDZ, Aug-ano-VTZ, Aug-ano-VQZ), and MP2/CBS(dAug-cc-pVDZ, dAug-cc-pVTZ, dAug-cc-pVQZ) levels of theory. Analysis of the calculation results indicates that incorporating corrections to the initial potential energy functions for relativistic effects and zero-point vibrational energy (ZPVE) is essential. The magnitude of the additional potential barrier arising from ZPVE was determined directly from anharmonic calculations, while the functional dependence of this correction on the torsional coordinate was obtained by scaling the corresponding function calculated in the harmonic approximation. Consideration of a parameter χ that is independent of the vibrational quantum numbers but depends on the molecular structural parameters also proved to be important. The calculated energies of stationary torsional states of the CH₃SH molecule at certain levels of theory and for relatively low torsional energies appeared to be in good agreement with experiment. Even better agreement between calculated and experimental data, extending over a wider energy range, was obtained for CH₃SD. The computed values of tunneling splitting of the ground vibrational states and the energies of torsional level of CD₃SH, CD₃SD, CH₃SH, and CH₃SH may be useful for analyzing their torsional-rotational spectra.