Highly-accurate quartic force fields for the prediction of anharmonic rotational constants and fundamental vibrational frequencies

•Quartic force fields of the highest level provide rotational constants to within, on average, 34 MHz of experiment.•Tetraatomic species with low anharmonicities can be within 20 MHz.•These same methods provide vibrational frequencies to within, on average, 5.8 cm−1. The CcCR quartic force field (QFF) methodology is capable of computing B0 and C0 rotational constants to within 35 MHz (0.14%) of experiment for triatomic and larger molecules with at least two heavy atoms. Additionally, the same constants for molecules with four or more atoms agree to within 20 MHz (0.12%) of experiment for the current test set. This work also supports previous claims that the same QFF methodology can produce fundamental vibrational frequencies with a deviation less than 5.7 cm−1 from experiment. Consequently, this approach of augmenting complete basis set extrapolated energies with treatments of core electron correlation and scalar relativity produces some of the most accurate rovibrational spectroscopic data available.