Approaching closed-shell accuracy for radicals using coupled cluster theory with perturbative triple substitutions

文献情報

出版日 2003-05-09

DOI 10.1039/B304542K

インパクトファクター 3.676

著者

Gregory J. O. Beran, Steven R. Gwaltney, Martin Head-Gordon

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要旨

The erratic performance of CCSD(T) for radicals is analyzed using non-Hartree–Fock references as a starting point for correlations and by testing the (2) approach as an alternative to (T) for including higher-order correlation effects. Though CCSD(2) improves upon CCSD(T), correlating from a better-behaved reference makes both theories robust. Comparisons of calculated harmonic frequencies against experiment in a set of diatomic radicals from Brueckner-like orbitals demonstrate improvement approaching closed-shell accuracy. Additionally, we find that using BLYP Kohn–Sham orbitals yields similar improvements, and they are therefore a useful, inexpensive reference for high-level correlation methods in difficult systems. Root-mean-square errors of 1.0–1.2% are found in the cc-pVQZ basis for predicted harmonic frequencies in the test set using OD(T) and KS-CCSD(T), making these approaches quite competitive with CCSD(T) for closed-shell molecules. Finally, these improvements are correlated with spin contamination and the rate of change of the electron density with nuclear displacement.

掲載誌

Physical Chemistry Chemical Physics

CiteScore: 5.5

自己引用率: 10.3%

年間論文数: 3036

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