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Issue 12, 2003
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Approaching closed-shell accuracy for radicals using coupled cluster theory with perturbative triple substitutions

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Abstract

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.

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Article information


Submitted
21 Feb 2003
Accepted
24 Apr 2003
First published
09 May 2003

Phys. Chem. Chem. Phys., 2003,5, 2488-2493
Article type
Paper

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

G. J. O. Beran, S. R. Gwaltney and M. Head-Gordon, Phys. Chem. Chem. Phys., 2003, 5, 2488
DOI: 10.1039/B304542K

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