F12-TZ-cCR: A Methodology for Faster and Still Highly Accurate Quartic Force Fields

Abstract

The F12-TZ-cCR quartic force field (QFF) methodology, defined here as CCSD(T)-F12b/cc-pCVTZ-F12 with further corrections for relativity, is introduced as a cheaper and even more accurate alternative to more costly composite QFF methods like those containing complete basis set extrapolations within canonical coupled cluster theory. F12-TZ-cCR QFFs produce B 0 and C 0 vibrationally averaged principal rotational constants within 7.5 MHz of gas-phase experimental values for tetraatomic and larger molecules, offering higher accuracy in these constants than the previous composite methods. In addition, F12-TZ-cCR offers an order of magnitude decrease in the computational cost of highly accurate QFF methodologies accompanying this increase in accuracy. An additional order of magnitude in cost reduction is achieved in the F12-DZ-cCR method, while also matching the accuracy of the traditional composite method's B 0 and C 0 constants. Finally, F12-DZ and F12-TZ are benchmarked on the same test set, revealing that both methods can provide anharmonic vibrational frequencies that are comparable in accuracy to all three of the more expensive methodologies, although their rotational constants lag behind. Hence, the present work demonstrates that highly accurate theoretical rovibrational spectral data can be obtained for a fraction of the cost of conventional QFF methodologies, extending the applicability of QFFs to larger molecules.

Anharmonic Vibrational Frequencies of Water Borane and Associated Molecules

“…While F12-TZ-cCR exhibits fairly large deviations from the experimental data for borane giving an overall MAE of 218.3 MHz, F12-TZ performs much worse with an MAE of 750.2 MHz. This is in line with previous work on both F12-TZ [ 22 , 23 , 24 ] and F12-TZ-cCR [ 28 ], which demonstrates that accounting for the effects of core correlation in F12-TZ-cCR is necessary for producing more accurate rotational constants. In the present case, neither of the methodologies utilized herein seems to be achieving real accuracy, but F12-TZ-cCR still has a clear advantage.…”

“…The F12-TZ-cCR values for both and agree to within just over 0.5 MHz. Such exceptional agreement even exceeds the expected performance of F12-TZ-cCR, which previously achieved an average agreement of roughly 7.5 MHz on similar and rotational constants [ 28 ]. This suggests that F12-TZ-cCR is very well suited to the determination of the vibrational spectrum of borinic acid and for the elucidation of both the rotational and vibrational spectra of water borane.…”

“…Chief among these composite methods is CcCR, which is composed of a complete basis set extrapolation (“C”), corrections for core correlation (“cC”), and corrections for scalar relativity (“R”) [ 20 ]. However, recent work [ 28 ] has explored the use of a hybrid between F12-TZ and CcCR, fittingly referred to as F12-TZ-cCR. This method utilizes CCSD(T)-F12b with the cc-pCVTZ-F12 basis set, explicit treatment of core electrons, and the same correction for scalar relativity as CcCR.…”

“…This method utilizes CCSD(T)-F12b with the cc-pCVTZ-F12 basis set, explicit treatment of core electrons, and the same correction for scalar relativity as CcCR. It offers more accurate rotational constants with agreement on the order of 7.5 MHz with experimental data while still capturing an order-of-magnitude decrease in the computational cost relative to CcCR [ 28 ]. As such, this methodology is also employed herein to offer better predictions of the rotational spectra of these molecules.…”

Confirmation of gaseous methanediol from state-of-the-art theoretical rovibrational characterization

“…High level QFFs based on coupled cluster theory [27][28][29] produce anharmonic vibrational frequencies that are often within 5 cm À1 of experiment and 30 MHz for rotational constants. [30][31][32][33][34][35][36][37][38][39][40][41][42] In this work the newly developed F12-TZ-cCR and F12-DZ-cCR QFFs, which produce exceptional accuracies of better even than 1 cm À1 for vibrational frequencies in many cases, 43 are utilized to attain this high accuracy for modest computational cost. The spectroscopic data provided by this work will also aid in possible future detections of methanediol in laboratory synthesis or in the interstellar medium.…”