F12+DFT Quartic Force Fields for Cost-Effective Theoretical Spectroscopy

CCSD(T)-F12 and DFT calculations are combined to compute accurate spectroscopic data for a set of benchmark molecules within 1.82% error of a high-level benchmarking method for vibrational frequencies. This is done by combining 3^rd order and 4^th order DFT force constants with 2^nd order CCSD(T)-F12 force constants in order to produce a “cheap + expensive” method. This approach can be applied to larger molecules where CCSD(T)-F12 single-point energies would take an infeasible amount time to calculate. Benchmark comparisons are made with the F12cCR approach with a triple zeta level basis set. This methodology is also used to calculate accurate spectroscopic data for several electronically excited states using time-dependent density functional theory combined with equation of motion coupled cluster techniques.