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AbstractsClosed form one centre partial wave perturbation results are obtained through secondorder in the energy for H$-like molecules with arbitrary nuclear charges. The expansion centre for the method is taken at an arbitrary point along the internuclear axis and the zeroth-order wave function is a screened "1s" function centred at the expansion point. Various fixed one centre calculations of other workers for the lsa states of H a and HeH++ and for the 2pa state of HeH* are generated as limiting cases of this more general treatment.The floating one centre perturbation results for the lsa states of H a and HeHff are used as models for discussing the usefulness of allowing the expansion centre in the one centre method to be a function of nuclear configuration.

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AbstractsVarious floating one-center perturbation schemes are developed for H$-like molecules. Previous work for the "Is" hydrogen atom approach is extended through third order in the energy. In rhe molecular puff approach an exact "closed form" solution is obtained for the zeroth-order problem, which corresponds to a double layer molecular puff. Semianalytical results for the first-order correction to the floating molecular puff wave function and for the energy through third order are developed which offer a convenient alternative to previous approaches. Explicit calculations for ground state H$ show that for R > 3a0 the "1s'' and the more complicated "puff" treatments are equivalent and that neither the "1s" nor the molecular puff function is an adequate zeroth-order wave function for the important intermediate values of R. The usefulness of some of these methods in a variety of problems, where one-center zeroth-order wave functions do appear to be adequate, is discussed briefly.On a dtveloppt des schtmas de perturbation difftrents qui emploient des fonctions d'onde A un centre flottant pour des moltcules de type Hg. Les travaux anttrieurs sur le proctdt "d'atome d'hydrogkne Is" ont t t t Ctendus jusqu'au troisikme ordre de l'knergie. Dans le proctdt de "puff" moltculaire on a obtenu une solution exacte dans une forme analytique finie pour le probltme dordre ztro, ce qui correspond A un ''puff:' moltculaire A doubles couches. Des rtsultata semi-analytiques pour la correction du premier ordre B cette fonction, et jusqu'au troisikme ordre pour l'tnergie ont t t t obtenus, ce qui offre une alternative utile a u proctdts employts anttrieurement. Des calculs explicites pour l'ttat normal de H$ montrent que pour R > 3a0 , les proctdts "1s" et "puff" sonttquiva- 120 ALI AND MEATHOrdnung erhalten, was einem "molecular puff" mit Doppelschicht entspricht. Semianalytische Resultate fur die Korreletur erster Ordnung zur Wellenfunktion mit fliessendem "molecular puff" und fur die Energie bis zur dritten Ordnung werden erhalten, was eine angemessene Alternative zu den friiheren Verfahren darbietet. Explizite Berechnungen fur den Grundzustand von H.$ zeigen, dass die "lS"-und "puff"-Verfahren fur R > 3s,, aquivalent sind und dass weder die " 1s"-noch die "puff" -Funktion eine geeignete Wellenfunktion nullter Ordnung fur die wichtigen intermediaren Werte von R ist. Die Anwendbarkeit einiger dieser Methoden in einer Menge von Problemen, wo Einzentrumfunktionen geeignet scheinen, wird kurz diskutiert.