Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 5, Issue 12, Pages 2488-2493Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/b304542k
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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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