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The Inner-Shell Correlation Component of TAE

Inner-shell correlation is a substantial part of the absolute correlation energy even for late first-row systems for second-row systems, it in fact rivals the absolute valence correlation energy in importance. However, its relative contribution to molecular TAEs is fairly small in benzene, for instance, it amounts to less than 0.7 % of the TAE. Even so, at 7 kcal/mol, its contribution is important by any reasonable thermochemical standard. By the same token, a 1 % relative error in a 7 kcal/mol contribution is tolerable even by benchmark thermochemistry standards, while the same relative error in a 300 kcal/mol contribution would be unacceptable even by the chemical accuracy standards. [Pg.40]

In addition, for thermochemical purposes we are primarily interested in the core-valence correlation, since we can reasonably expect the core-core contributions to largely cancel between the molecule and its constituent atoms. (The partitioning between core-core correlation -involving excitations only from inner-shell orbitals - and core-valence correlation - involving simultaneous excitations from valence and inner-shell orbitals - was first proposed by Bauschlicher, Langhoff, and Taylor [42]). [Pg.40]

For these reasons, we feel justified in treating the inner-shell correlation contribution to TAE as a separate contribution, rather than together with the valence correlation. There are substantial cost advantages to this rather than having to carry out very elaborate all-electrons-correlated CCSD(T) calculations in basis sets near saturation for both valence and inner-shell correlation, we can limit these costly calculations to a basis set that is primarily saturated for inner-shell correlation. [Pg.40]

A tentative explanation for the importance of connected triple excitations for the inner-shell contribution to TAE can be found in the need to account for simultaneously correlating a valence orbital and relaxing an inner-shell orbital, or conversely, requiring a double and a single excitation simultaneously. [Pg.41]

In principle, one could contract at least the few innermost s primitives and reduce the basis set further. By leaving the basis set completely uncontracted, however, we can recycle the integrals and SCF wavefunc-tion for the next step of the calculation. [Pg.41]


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