Assessment of DensityFunctional Theory for ThermochemicalApproaches Based on Bond Separation Reactions.

The recently proposed ATOMIC protocol is a fully ab initiothermochemicalprotocol that rests upon the concept of bond separation reactions(BSRs) to correct for systematic errors of composite wave functionapproaches. It achieves high accuracy for atomization energies andderived heats of formation if basis set requirements for all contributingcomponents are balanced carefully. The present work explores the potentialof density functionals as possible replacements of composite wavefunction approaches in the ATOMIC protocol. Twenty density functionalsare examined for their accuracy in thermochemical predictions basedon calculated bond-separation energies and precomputed high-leveldata for the small parent molecules entering BSRs. The best densityfunctionals outperform CCSD (coupled cluster with singles and doublesexcitations), but none reaches the accuracy of well-balanced compositewave function approaches that consider quasiperturbational connectedtriples excitations at least with small basis sets. Some functionalsshow unexpected problems with bond separation reactions and are analyzedfurther with a model of empirically calibrated bond additivity corrections.Finally, the benefit of adding empirical dispersion terms to commondensity functionals is analyzed in the context of BSR-corrected thermochemistry. [ABSTRACT FROM AUTHOR]