Extended benchmark studies of coupled cluster theory through triple excitations

Coupled cluster theory through quasiperturbative triple excitations CCSD(T) was used with large correlation consistent basis sets to obtain optimized structures, harmonic vibrational frequencies and atomization energies for 37 molecules from the G2/97 test set. In some cases, it proved possible to include the triple excitations iteratively via CCSDT. Use of various correlation consistent basis set sequences facilitated estimation of frozen core energies in the complete basis set limit. Tight d functions were added for all second row atoms in order to improve the basis set convergence properties. Core/valence correlation corrections were obtained from all electron CCSD(T)/cc-pCVQZ calculations. Scalar relativistic contributions to the atomization energy were obtained from configuration interaction mass-velocity/one-electron Darwin calculations and CCSD(T) Douglas–Kroll–Hess calculations. By combining results from the present work with previously reported findings, a total of 114 comparisons with reliable experimental data for molecular atomization energies were possible. A statistical analysis of the level of agreement with experiment was performed, leading to a mean absolute deviation of 0.8 kcal/mol and maximum absolute error of −4.4 kcal/mol. This represents the most thorough study to date of the reliability of a composite approach to computational thermochemistry based on coupled cluster theory. The approach avoids the use of additivity approximations to estimate the complete basis set limit and does not include empirical corrections to the electronic energy. Results from three parameterized methods (G2, G3, and CBS-Q) for the same set of molecules are compared to the coupled cluster results.