Probing the limits of accuracy in electronic structure calculations: Is theory capable of results uniformly better than "chemical accuracy"?

Current limitations in electronic structure methods are discussed from the perspective of their potential to contribute to inherent uncertainties in predictions of molecular properties, with an emphasis on atomization energies (or heats of formation). The practical difficulties arising from attempts to achieve high accuracy are illustrated via two case studies: the carbon dimer (C-2) and the hydroperoxyl radical (HO2). While the HO2 wave function is dominated by a single configuration, the carbon dimer involves considerable multiconfigurational character. In addition to these two molecules, statistical results will be presented for a much larger sample of molecules drawn from the Computational Results Database. The goal of this analysis will be to determine if a combination of coupled cluster theory with large 1-particle basis sets and careful incorporation of several computationally expensive smaller corrections can yield uniform agreement with experiment to better than "chemical accuracy" (+/- 1 kcal/mol). In the case of HO2, the best current theoretical estimate of the zero-point-inclusive, spin-orbit corrected atomization energy (Sigma D-0=166.0 +/- 0.3 kcal/mol) and the most recent Active Thermochemical Table (ATcT) value (165.97 +/- 0.06 kcal/mol) are in excellent agreement. For C-2 the agreement is only slightly poorer, with theory (D-0=143.7 +/- 0.3 kcal/mol) almost encompassing the most recent ATcT value (144.03 +/- 0.13 kcal/mol). For a larger collection of 68 molecules, a mean absolute deviation of 0.3 kcal/mol was found. The same high level of theory that produces good agreement for atomization energies also appears capable of predicting bond lengths to an accuracy of +/- 0.001 angstrom.