F12c: A New Methodology for Highly- Accurate Quartic Force Fields

The F12c-TZ-cCR quartic force field (QFF) methodology, defined here as CCSD(T)-F12c/cc-pCVTZ-F12 with further corrections for relativity, 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. F12c-TZ-cCR QFFs produce B₀ and C₀ vibrationally-averaged principal rotational constants within 21.4 MHz (0.13%) of gas-phase experimental values for tetratomic and larger molecules and anharmonic vibrational frequencies within 11.3 cm^-1 (0.82%) of experiment. The faster F12c-DZ-cCR QFFs produce B₀ and C₀ vibrationally-averaged principal rotational constants within 14.6 MHz (0.15%) and anharmonic vibrational frequencies within 13.6 cm^-1 (1.54 %) of experiment. Comparatively F12b-DZ-cCR QFFs, which is 23 percent faster than F12c-DZ-cCR, produce rotational constants within 21.9 MHz (0.18%) and anharmonic vibrational frequencies withing 20.0 cm^-1 (2.37 %) of experiment. The present work demonstrates that highly accurate theoretical rovibrational spectral data can be obtained through F12c QFFs.