The Hartree-Fock-Heitler-London and the Chemical Orbital Methods: Theory and Computational Verification

摘要

For chemistry the theoretical representation of the forces connecting atoms in molecules was and is a central problem. We have briefly surveyed the history of western world chemistry, to allow a deeper appreciation on the evolution of chemical theories for the constitution of matter. Then we have concentrated the discussion on the Atomic and the Molecular Orbital, the basic building blocks in the Heitler-London, HL, and in the Linear Combination of Atomic Orbitals-Molecular Orbital, LCAO-MO, methods, which have lead to the construction of modern Valence Bond and Hartree- Fock methods (and related extensions). As it is known, accurate predictions from non semiempirical methods often require enormous amount of computer power, if applied to molecules of reasonable size and current chemical interest. Merging of the Hartree-Fock with the Heitler-London algorithms, as recently proposed in the Hartree-Fock-Heitler-London, HF-HL, method, reduces the length of the expansions needed in AO or MO ab initio models; this simplification allows easy interpretation of the resulting wave function. The HF-HL method is exemplified with systematic computations on ground and excited state of the hydrides and homonuclear diatomic molecules with atoms of the first and second period of the periodic table. Further, we show that the HF-HL method is derivable from a wave function constructed with a new type of orbital, the Chemical Orbital, which embodies the characterization of MO near equilibrium, AO at dissociation and at the united atom. Preliminary computations with Chemical Orbitals are included. The new method provides the conceptual origin of both the HF and VB approaches, thus the foundation of the last eighty years effort in variational quantum chemistry.