Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Polishing the Gold Standard: The Role of Orbital Choice in CCSD(T)
  Vibrational Frequency Prediction   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 spin-unrestricted 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 M{\o}ller-Plesset perturbation theory
($\kappa$-OOMP2) orbitals as reference orbitals for MP3, we investigate the use
of $\kappa$-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 neutrals, 15 cations, 6 anions). The use of
$\kappa$-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 $\kappa$-OOMP2 orbital reference
(CCSD(T):$\kappa$-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 $\kappa$-OOMP2 orbitals has also proven
useful in diagnosing multireference character that can hinder the reliability
of CCSD(T).

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The categories of this paper are: solv-int
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The categories of this paper are: gr-qc
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The categories of this paper are: ao-sci
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The categories of this paper are: cond-mat
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The categories of this paper are: chao-dyn
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The categories of this paper are: q-alg
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The categories of this paper are: atom-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: hep-ph
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The categories of this paper are: adap-org
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The categories of this paper are: cs
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The categories of this paper are: astro-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: mtrl-th
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The categories of this paper are: patt-sol
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The categories of this paper are: plasm-ph