The performance of explicitly correlated methods for the computation of anharmonic vibrational frequencies

Abstract

Anharmonic vibrational frequencies for closed‐shell molecules computed with CCSD(T)‐F12b/aug‐cc‐pVTZ differ from significantly more costly composite energy methods by a mean absolute error (MAE) of 7.5 cm−1 per fundamental frequency. Comparison to a few available gas phase experimental modes, however, actually lowers the MAE to 6.0 cm−1. Open‐shell molecules have an MAE of nearly a factor of six greater. Hence, open‐shell molecular anharmonic frequencies cannot be as well‐described with only explicitly correlated coupled cluster theory as their closed‐shell brethren. As a result, the use of quartic force fields and vibrational perturbation theory can be opened to molecules with six or more atoms, whereas previously such computations were limited to molecules of five or fewer atoms. This will certainly assist in studies of more chemically interesting species, especially for atmospheric and interstellar infrared spectroscopic characterization.

A Molecular Candle Where Few Molecules Shine: HeHHe+

“…3 B 1 He 2 H + will also be rotationally active due to its atomic arrangement and modest dipole moment (0.83 D), and the full set of rotational constants and vibrational frequencies are given in Table 1. Benchmarks for CCSD(T)-F12/aug-cc-pVTZ for He 2 + compared with derived experimental data [44] are quite good in accordance with recent vibrational benchmarks for this method [39][40][41]. Furthermore, the CCSD(T)-F12/aug-cc-pVTZ values are nearly identical to those computed with the composite method utilized previously [15].…”

“…The relative energies are computed from the energies of the CCSD(T)-F12/aug-cc-pVTZ optimized geometries. CCSD(T)-F12 at the triple-ζ basis level has recently been shown to produce vibrational frequencies often within less than 5 cm −1 of much higher levels of theory including considerations made for complete basis set extrapolations, scalar relativity, and core electron correlation [39][40][41]. For these molecules, relativity for the electronic structure will be insignificant, and there are no core electrons.…”

Pathways to Detection of Strongly-Bound Inorganic Species: The Vibrational and Rotational Spectral Data of AlH2OH, HMgOH, AlH2NH2, and HMgNH2

“…The other method is a composite scheme based on canonical CCSD(T) but with considerations for complete basis set (CBS) limit extrapolations ("C"), core electron correlation ("cC"), and relativity ("R") to give the so-called CcCR QFF (Fortenberry et al, 2011). The F12-TZ QFF has been shown to produce fundamental anharmonic vibrational frequencies nearly coincident with the much more costly CcCR method (Agbaglo et al, 2019;Agbaglo and Fortenberry, 2019a;Agbaglo and Fortenberry, 2019b), but the composite method is still required for accurate rotational constants (Gardner et al, 2021). The F12-TZ QFF will be utilized on all four molecules of the present study.…”

Anharmonic Vibrational Frequencies of Water Borane and Associated Molecules

“…QFFs are fourth-order Taylor series expansions of the internuclear potential energy portion of the Watson Hamiltonian [ 20 ]. When coupled with the F12-TZ level of theory, QFFs frequently offer agreement with gas-phase vibrational frequencies of within 5 to 7 cm [ 21 , 22 , 23 , 24 , 25 ]. Other techniques for computing accurate anharmonic spectral data exist [ 26 ], but as a result of the good performance of the F12-TZ QFF on ammonia borane, this same methodology is used herein to investigate water borane (BH OH ), borinic acid (BH OH), HBO, and borane (BH ).…”

“…One shortcoming of the F12-TZ methodology is its inability to produce very accurate rotational constants [ 22 , 23 , 24 ]. When accurate rotational data is needed, much more expensive composite QFFs have often been employed that achieve agreement of about 20 MHz in the vibrationally-averaged ground state rotational constants [ 27 ].…”