AbstractsGroup-theoretical methods for determining permitted terms of electrons in polyatomic molecules and complexes in the MO and HL methods are proposed. Essentially, the permutational symmetry of the electron system is utilized. Some concrete examples of the operation of these methods are considered.On propose des mtthodes provenantes de la theorie des groupes pour dtterminer les termes d'tlectrons permis dans les molecules et les complexes polyatomiques dans le cadre des methodes MO et HL. On utilise principalement la symktrie permutationnelle du syst6me d'klectrons. On examine quelques exemples concrets pour illustrer les mtthodes propostes.Gruppentheoretische Methoden fur die Bestimmung der in mehratomigen Molekulen und Komplexen erlaubten Elektronenterme werden vorgeschlagen; diese Methoden konnen sowohl in der MO-als auch in der HL-Methode angewendet werden. Man benutzt wesentlich die Permutationssymmetrie der Elektronensysteme. Einige konkrete Beispiele werden betrachtet.

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The crystal field theory describes the interelectronic interaction of an impurity 3d-ion by the Slater parameters F F , F similar to the free ion case. However, 0'2 4 the quantitative explanation of the optical spectra implies a modification of these parameters relative to the free ion case. This modification depends on the nature of the ligands (1 to 3).In recent years numerous proofs of the partial delocalisation of the electrons of the impurity ion have appeared describing their ljehaviour by molecular orbitals (MO) rather than atomic ones (AO)(partially covalent bonds) (4 to 9).The change of the Slater parameters was theoretically considered in (1, 10 to 12). However, in the all these papers the electron charge transfer (covalency) was not taken into account. Taking into account these covalent effects we have calculated the energy spectrum of an impurity ion with 3d -configuration in octahedral configuration. We have found that covalency, generally speaking, leads to new parameters. 2The bonding MO' s are usually closed and therefore lead only to a total shift m n e 2g g of all energy levels of each configuration t of the antibonding electrons. The antibonding electron interactions (correlation) produce the splitting of these levels.Forming the exact wave functions of the two-electron system for all a Sr-terms of 2 the t2 t e e configurations from the t and e MO' s and taking into account "the term interaction" of the same type (by influence of the correlation 2g' 2g g' g 2gf g 2 Vee = e /r case of the "strong field scheme!!. The matrix elements ( a S r l V e e I d S r > of this equation can be expressed through 10 two-particle Coulomb and exchange integrals (13). Unlike the crystal field theory, the MO LCAO treatment does not relate to one centre and is not characterized by radial and angular parts, a s in the case of a spherically symmetric field. Therefore, the question if it is possible to ), one can obtain a secular equation of 10-th order, analogous to the 12

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