Prediction of Reaction Barriers and Thermochemical Properties with Explicitly Correlated Coupled-Cluster Methods: A Basis Set Assessment

We assessed the performance of our perturbative explicitly correlated coupled-cluster method, CCSD­(T)_F12, for accurate prediction of chemical reactivity. The reference data included reaction barrier heights, electronic reaction energies, atomization energies, and enthalpies of formation from the following sources: (1) the DBH24/08 database of 22 reaction barriers (Truhlar et al.), (2) the HJO12 set of isogyric reaction energies (Helgaker et al.), and (3) a HEAT set of atomization energies and heats of formation (Stanton et al.). We performed two types of analyses targeting the two distinct uses of explicitly correlated CCSD­(T) models: as a replacement for basis-set-extrapolated CCSD­(T) in highly accurate composite methods like HEAT and as a distinct model chemistry for standalone applications. Hence, we analyzed in detail (1) the <i>basis set error</i> of each component of the CCSD­(T)_F12 contribution to the chemical energy difference in question and (2) the <i>total error</i> of the CCSD­(T)_F12 model chemistry relative to the benchmark values. Two basis set families were utilized in the calculations: the standard aug-cc-p­(C)­VXZ-F12 (X = D, T, Q) basis sets for the conventional correlation methods and the cc-p­(C)­VXZ-F12 (X = D, T, Q) basis sets of Peterson and co-workers that are specifically designed for explicitly correlated methods. Our conclusion is that the performance of the two families for CCSD correlation contributions (which are the only components affected by the explicitly correlated terms in our formation) are nearly identical with triple- and quadruple-ζ quality basis sets, with some differences at the double-ζ level. Chemical accuracy (∼4.18 kJ/mol) for reaction barrier heights, electronic reaction energies, atomization energies, and enthalpies of formation is attained on average with the aug-cc-pVDZ, aug-cc-pVTZ, cc-pCVTZ-F12/aug-cc-pCVTZ, and cc-pCVDZ-F12 basis sets, respectively, at the CCSD­(T)_F12 level of theory. The corresponding mean unsigned errors are 1.72 kJ/mol, 1.5 kJ/mol, ∼2 kJ/mol, and 2.17 kJ/mol, and the corresponding maximum unsigned errors are 4.44 kJ/mol, 3.6 kJ/mol, ∼5 kJ/mol, and 5.75 kJ/mol