Computing Accurate & Reliable Rovibrational Spectral Data for Aluminum-Bearing Molecules

The difficulty of quantum chemically computing vibrational, rotational, and rovibrational reference data via quartic force fields (QFFs) for molecules containing aluminum appears to be alleviated herein using a hybrid approach based upon CCSD(T)-F12b/cc-pCVTZ further corrected for conventional CCSD(T) scalar relativity within the harmonic terms and simple CCSD(T)-F12b/cc-pVTZ for the cubic and quartic terms: the F12-TcCR+TZ QFF. Aluminum containing molecules are theorized to participate in significant chemical processes in both the Earth's upper atmosphere as well as within circumstellar and interstellar media. However, experimental data for the identification of these molecules are limited, showcasing the potential for quantum chemistry to contribute significant amounts of spectral reference data. Unfortunately, current methods for the computation of rovibrational spectral data have been shown previously to exhibit large errors for aluminum-containing molecules. In this work, ten different methods are benchmarked to determine a method to produce experimentally-accurate rovibrational data for theorized aluminum species. Of the benchmarked methods, the explicitly correlated, hybrid F12-TcCR+TZ QFF consistently produces the most accurate results compared to both gas-phase and Ar-matrix experimental data. This method combines the accuracy of the composite F12-TcCR energies along with the numerical stability of non-composite anharmonic terms where the non-rigid nature of aluminum bonding can be sufficiently treated.