Performance of a Hybrid and F12*/F12c explicitly correlated coupled cluster method for the use in anharmonic vibrational frequency computations

A hybrid quartic force field (QFF) approach that produces the same accuracies as non-hybrid methods but for less than one quarter of the computational time is introduced as an alternative to calculate highly accurate theoretical rovibrational spectral data for application to observations from both ground-based radio telescopes and space-based missions like JWST. This method utilizes explicitly correlated coupled cluster theory at the singles and doubles level inclusive of perturbative triples (CCSD(T)-F12b) in conjunction with a triple-zeta basis set, core electron correlation, and scalar relativity for the harmonic terms and CCSD(T)-F12b with a valence double- basis set for the cubic and quartic terms. There is no sacrifice in the prediction of fundamental anharmonic vibrational frequencies or vibrationally-averaged rotational constants as compared to experiment, but take on average 10% of the time as the previously used method. F12c is also tested, but as an increased computational time for no increased in accuracy. As such, this work reports a hybrid approach (F12-TcCR+DZ) and F12c in the computation of rovibrational spectral data which can be applied to the observation of novel molecules in the gas phase in the laboratory and potentially even in astrophysical environments.