Accurate thermochemistry for medium-sized and large molecules

Accurate techniques such as Gaussian-2 (G2) theory have been proposed in recent years to evaluate the thermochemistry of small molecules from first-principles. However, as the molecules get larger, the errors in G2 theory and similar approaches tend to accumulate. For example, the computed heats of formation of benzene and naphthalene with G2 and G2(MP2) theories, respectively, have errors of 3.9 and 7.2 kcal/mol. In this work, we explore strategies for computing accurate heats of formation for medium-sized and large molecules. In our first scheme, G2 theory is combined with isodesmic bond separation reaction energies to yield accurate thermochemistry for larger molecules. For a test set of 40 molecules composed of H, C, O, and N, our method yields enthalpies of formation, {Delta}H{sub f}{sup 0}(298 K), with a mean absolute deviation from experiment of only 0.5 kcal/mol. This is an improvement of a factor of three over the deviation of 1.5 kcal/mol seen in standard G2 theory.