Some remarks on the Pariser-Parr-Pople method

Basic assumptions which characterize the Pariser-Parr-Pople method of computing molecular electronic wave functions are critically examined. By restricted variational calculation of the valence state of carbon and nitrogen atoms and ions, it is demonstrated that the usual methods of evaluation of one-centre Coulomb integrals and atomic core energies are rather good. A semi-theoretical means of estimating the core resonance integral is proposed and shown to give fair agreement with the empirical values for C-C, O-O, C-N, and C-O bonds.

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Zusammenfassung Die Grundannahmen der Methode von Pariser-Parr-Pople zur Berechnung molekularer Wellenfunktionen werden kritisch durchleuchtet. Mittels beschränkter Variationsansätze für den Valenzzustand von Kohlenstoff- und Stickstoff-Atomen und -Ionen wird dargelegt, daß die Methoden, wie sie üblicherweise zur Berechnung von Einzentren-Coulombintegralen und atomaren Rumpfenergien angewendet werden, zu recht guten Ergebnissen führen. Ferner wird ein halbtheoretisches Verfahren zur Abschätzung der Rumpfresonanzintegrale vorgeschlagen und nachgewiesen, daß das Resultat gut mit den empirischen Werten für die C-C-, O-O-, C-N- und C-O-Bindung übereinstimmt.

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Résumé Les hypothèses fondamentales de la méthode de Pariser-Parr-Pople pour le calcul des fonctions d'onde électroniques sont examinées critiquement. Un calcul variationel limité des états de valence des atoms et ions de carbone et de nitrogène montre que les méthodes usuelles pour l'évaluation des intégrales de Coulomb monocentriques et des énergies de coeur atomiques sont assez bonnes. Nous proposons un procédé semithéorique pour évaluer l'intégrale de résonance de coeur, qui s'accorde assez bien aux valeurs expérimentales pour les liaisons C-C, O-O, C-N et C-O.

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The research reported in this paper was sponsored in part by the King Gustaf VI Adolf's 70-Years Fund for Swedish Culture, Knut and Alice Wallenberg's Foundation, the Swedish Natural Science Research Council, and in part by the Aeronautical Research Laboratory, OAR, through the European Office, Aerospace Research, United States Air Force, and also in part by the National Aeronautics and Space Administration Research Grant NsG-512.

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On leave of absence from the Department of Chemistry, Faculty of Science, Hokkaido, University, Sapporo, Japan.     

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