Commentationes Localized Bond Orbitals and the Correlation Problem I. The Perturbation Calculation of the Ground State Energy

A method is proposed to go beyond the SCF result in the calculation of the ground state energies without any variational procedure. One chooses a set of reasonable bonding and antibonding orbitals localized on the chemical bonds. The bonding orbitals are used to built a fully localized determinant. The basis of excited states is built using the antihonding orbitals. One calculates the lower eigenvalue of the CI matrix in this basis by a Rayleigh-Schr6dinger expansion. The conceptual and practical advantages of the method are discussed, and the perturbation series is specified in order to satisfy the linked cluster theorem conditions and to retain the advantages of the Epstein-Nesbet partition of the Hamiltonian. Es wird eine Methode angegeben, die die SCF-Resultate in der Berechnung der Grundzustandsenergien tibertrifft, ohne die Mittel der Variationsrechnung anzuwenden. Als Basisfunktionen w~hlt man einen Satz geeigneter bindender and antibindender Orbitale, die auf den chemischen Bindungen lokalisiert sind. Die bindenden Orbitale werden zum Aufbau einer v611ig lokalisierten Determinante benutzt. Als Basisfunktionen ftir die angeregten Zust[inde benutzt man auch antibindende Orbitale. Man berechnet den tieferen Eigenwert der CI Matrix mit dieser Basis durch eine Rayleigh-Schr6dingerEntwicklung. Die begriffiichen und praktischen Vorteile dieser Methode werden diskutiert. Indem man die St~Srungsreihe so w~ihlt, dab das "linked cluster"-Theorem erftillt ist, bleiben die Vorteile der Epstein-Nesbet-Aufspaltung des Hamiltonoperators erhalten.

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