Effects of increased basis-set size as well as a correlated treatment of the diagonal Born-Oppenheimer approximation are studied within the context of the high-accuracy extrapolated ab initio thermochemistry (HEAT) theoretical model chemistry. It is found that the addition of these ostensible improvements does little to increase the overall accuracy of HEAT for the determination of molecular atomization energies. Fortuitous cancellation of high-level effects is shown to give the overall HEAT strategy an accuracy that is, in fact, higher than most of its individual components. In addition, the issue of core-valence electron correlation separation is explored; it is found that approximate additive treatments of the two effects have limitations that are significant in the realm of theoretical thermochemistry.
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It is important to realize here that this statement refers to total electronic energies, i.e., which is equal to the sum of successive ionization potentials. This, of course, is even more demanding than the total atomization energies, as the error cancellation between nonbonding and, especially, core electrons in an atomization energy scheme is not operative. This is why the estimated error bar is so much greater than that associated with TAEs.
All ATcT results reported in this paper are based on the Core (Argonne) Thermochemical Network Version 1.064, 2007.
Technically, this is not an ab initio result, as is the HEAT456-QP value of , since experimental information has been used. It is, nonetheless, the best result that can be determined by “theory,” as the other contributions to the atomization energy used in the HEAT approach are not experimental observables.
This argument applies only to the electronic energy contribution and not to the zero-point energy, which presents a different set of problems.
