A cost-effective basis-set extrapolation scheme: Application to the energetics of homolytic bond dissociation

A composite procedure based on density functional theory (DFT) geometry optimizations and coupled cluster calculations with single and double excitations and perturbative treatment of triple excitations (CCSD(T)) is proposed for the evaluation of homolytic bond dissociation enthalpies (BDEs). The performance of several functionals for predicting the structure and vibrational frequencies of a selected set of closed- and open-shell species was investigated. By using the correlation consistent cc-pVTZ basis-set, it was found that B3LYP and VSXC geometries are in good agreement with experiment. B3LYP/cc-pVTZ geometries were then selected for CCSD(T) single-point energy calculations. The Hartree-Fock (HF) contribution to the total energy was estimated at the HF/cc-pV6Z level and also by using a (x = D(2),T(3),Q(4),5,6) ad hoc extrapolation. Complete basis-set values for CCSD(T) correlation energies were evaluated through dual (x, x + 1;x = 2) extrapolation schemes relying on calculations with the cc-pVxZ basis-set. The results illustrate the importance of the extrapolation schemes and show that (2, 3) extrapolated BDEs are more accurate than those calculated with the cc-pV6Z basis-set.