Hermitian “chemical” Hamiltonian: an alternative ab initio method
Some previous results of the present author are combined in order to develop a Hermitian version of the “Chemical Hamiltonian Approach.” In this framework the second quantized Born–Oppenheimer Hamiltonian is decomposed into one- and two-center components, if some fi nite basis corrections are omitted. (No changes are introduced into the one- and two-center integrals, while projective expansions are used for the three- and four-center ones, which become exact only in the limit of complete basis sets.) The total molecular energy calculated with this Hamiltonian can then presented as a sum of the intraatomic and diatomic energy terms which were introduced in our previous “chemical energy component analysis” scheme. The corresponding modifi ed Hartree–Fock–Roothaan equations are also derived; they do not contain any three- and fourcenter integrals, while the non-empirical character of the theory is conserved. This scheme may be useful also as a “layer” in approaches like ONIOM.
KeywordsChemical Hamiltonian Approach Alternative non-empirical SCF formalism Second quantized Hamiltonian Excluding three- and four-center integrals Projective integral approximation
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