Abstract
The CCSD, CCSD(T), and CR-CC(2,3) coupled cluster methods, combined with five triple-zeta basis sets, namely, MG3S, aug-cc-pVTZ, aug-cc-pV (T+d) Z, aug-cc-pCVTZ, and aug-cc-pCV (T+d) Z, are tested against the DBH24 database of diverse reaction barrier heights. The calculations confirm that the inclusion of connected triple excitations is essential to achieving high accuracy for thermochemical kinetics. They show that various noniterative ways of incorporating connected triple excitations in coupled cluster theory, including the CCSD(T) approach, the full CR-CC(2,3) method, and approximate variants of CR-CC(2,3) similar to the triples corrections of the CCSD(2) approaches, are all about equally accurate for describing the effects of connected triply excited clusters in studies of activation barriers. The effect of freezing core electrons on the results of the CCSD, CCSD(T), and CR-CC(2,3) calculations for barrier heights is also examined. It is demonstrated that to include core correlation most reliably, a basis set including functions that correlate the core and that can treat core-valence correlation is required. On the other hand, the frozen-core approximation using valence-optimized basis sets that lead to relatively small computational costs of CCSD(T) and CR-CC(2,3) calculations can achieve almost as high accuracy as the analogous fully correlated calculations.
| Original language | English (US) |
|---|---|
| Article number | 044108 |
| Journal | Journal of Chemical Physics |
| Volume | 128 |
| Issue number | 4 |
| DOIs | |
| State | Published - 2008 |
Bibliographical note
Funding Information:The authors are grateful to Steven Mielke for helpful discussion. The work at the University of Minnesota was supported in part by the U. S. Department of Energy, Office of Basic Energy Sciences, under Grant No. DE-FG02-86ER13579 (D.G.T.). The work at Michigan State University was supported by the U. S. Department of Energy, Office of Basic Energy Sciences, under Grant No. DE-FG02-01ER15228 (P.P.), and by the National Science Foundation’s Graduate Research Fellowship (J.R.G.). Some of the calculations were performed on the computer systems provided by the High Performance Computing Center at Michigan State University.