CCS Publication Abstract
Approaching Closedshell Accuracy for Radicals Using Coupled Cluster Theory with Perturbative Triple Substitutions
Beran, J. G. O., Gwaltney, S. R., & HeadGordon, M. (2003). Approaching Closedshell Accuracy for Radicals Using Coupled Cluster Theory with Perturbative Triple Substitutions. Physical Chemistry Chemical Physics. 5, 24882493.
Abstract
The erratic performance of CCSD(T) for radicals is analyzed using nonHartree–Fock references as a starting point for correlations and by testing the (2) approach as an alternative to (T) for including higherorder correlation effects. Though CCSD(2) improves upon CCSD(T), correlating from a betterbehaved reference makes both theories robust. Comparisons of calculated harmonic frequencies against experiment in a set of diatomic radicals from Bruecknerlike orbitals demonstrate improvement approaching closedshell accuracy. Additionally, we find that using BLYP Kohn–Sham orbitals yields similar improvements, and they are therefore a useful, inexpensive reference for highlevel correlation methods in difficult systems. Rootmeansquare errors of 1.0–1.2% are found in the ccpVQZ basis for predicted harmonic frequencies in the test set using OD(T) and KSCCSD(T), making these approaches quite competitive with CCSD(T) for closedshell molecules. Finally, these improvements are correlated with spin contamination and the rate of change of the electron density with nuclear displacement.
