Performance of coupled cluster theory in thermochemical calculations of small halogenated compounds

Atomization energies at 0 K and heats of formation at 298 K were obtained for a collection of small halogenated molecules from coupled cluster theory including noniterative, quasiperturbative triple excitations calculations with large basis sets (up through augmented septuple zeta quality in some cases). In order to achieve near chemical accuracy (±1 kcal/mol) in the thermodynamic properties, we adopted a composite theoretical approach which incorporated estimated complete basis set binding energies based on frozen core coupled cluster theory energies and (up to) five corrections: (1) a core/valence correction; (2) a Douglas–Kroll–Hess scalar relativistic correction; (3) a first-order atomic spin–orbit correction; (4) a second-order spin–orbit correction for heavy elements; and (5) an approximate correction to account for the remaining correlation energy. The last of these corrections is based on a recently proposed approximation to full configuration interaction via a continued fraction approximant for coupled cluster theory CCSD(T)-cf. Failure to consider corrections (1) to (4) can introduce errors significantly in excess of the target accuracy of ±1 kcal/mol. Although some cancellation of error may occur if one or more of these corrections is omitted, such a situation is by no means universal and cannot be relied upon for high accuracy. The accuracy of the Douglas–Kroll–Hess approach was calibrated against both new and previously published four-component Dirac Coulomb results at the coupled cluster level of theory. In addition, vibrational zero-point energies were computed at the coupled cluster level of theory for those polyatomic systems lacking an experimental anharmonic value.