Polishing the Gold Standard: The Role of Orbital Choice in CCSD(T) Vibrational Frequency Prediction

Abstract

While CCSD(T) with spin-restricted Hartree-Fock (RHF) orbitals has long been lauded for its ability to accurately describe closed-shell interactions, the performance of CCSD(T) on open-shell species is much more erratic, especially when using a spinunrestricted HF (UHF) reference. Previous studies have shown improved treatment of open-shell systems when a non-HF set of molecular orbitals, like Brueckner or Kohn-Sham density functional theory (DFT) orbitals, is used as a reference. Inspired by the success of regularized orbital-optimized second-order Møller-Plesset perturbation theory (κ-OOMP2) orbitals as reference orbitals for MP3, we investigate the use of κ-OOMP2 orbitals and various DFT orbitals as reference orbitals for CCSD(T) calculations of the corrected ground-state harmonic vibrational frequencies of a set of 36 closed-shell (29 neutrals, 6 cations, 1 anion) and 59 open-shell diatomic species (38 1 neutrals, 15 cations, 6 anions). The aug-cc-pwCVTZ basis set is used for all calculations. The use of κ-OOMP2 orbitals in this context alleviates difficult cases observed for both UHF orbitals and OOMP2 orbitals. Removing 2 multireference systems and 12 systems with ambiguous experimental data leaves a pruned data set. Overall performance on the pruned data set highlights CCSD(T) with a B97 orbital reference (CCSD(T):B97), CCSD(T) with a κ-OOMP2 orbital reference (CCSD(T):κ-OOMP2), and CCSD(T) with a B97M-rV orbital reference (CCSD(T):B97M-rV) with RMSDs of 8.48 cm −1 and 8.50 cm −1 , and 8.75 cm −1 respectively, outperforming CCSD(T):UHF by nearly a factor of 5. Moreover, the performance on the closed-and open-shell subsets show these methods are able to treat open-shell and closed-shell systems with comparable accuracy and robustness. The use of κ-OOMP2 orbitals has also proven useful in diagnosing multireference character that can hinder the reliability of CCSD(T).

Third order Møller-Plesset theory made more useful? The role of density functional theory orbitals

“…Specifically, it is interesting to consider whether DFT approximations with a low penchant for spin-contamination 43 and tunable delocalization error [44][45][46] can yield reasonable results. DFT orbitals have been shown to improve performance of CC approaches in many difficult cases [47][48][49][50] and it seems plausible that they would have an even larger impact on MP theory due to lack of iterative singles amplitudes (that mimic orbital relaxation in projected CC theories) in the latter. We consequently examine the performance of both scaled and unscaled MP2 and MP3 with DFT orbitals in this work, focusing on the ability to predict chemically relevant energy differences and dipole moments.…”