Electronic dipole and quadrupole polarizabilities and the Sternheimer shielding or antishielding factors are calculated for several closed-shell atomic systems. The calculations are carried out by an effective-field method using the perturbed orbitals, given by the uncoupled Hartree-Pock method, as trial functions. The external field is replaced either by a mean effective field taking into account the mean coupling between the deformations of the various orbitals or by a set of effective fields, each of them being relative to a particular perturbed orbital. Numerical results obtained for the He, Ne, A, and Kr isoelectronic series show a much better agreement with the corresponding coupled values than the initial uncoupled results.

Electronic multipolar polarizabilities of closed-shell isolated atomic systems are calculated by a variational method using, as trial functions, the perturbed wave functions given by the uncoupled Hartree-Fock method. The external field is replaced by an effective parametric field taking into account the coupling between the deformations of the various orbitals. Numerical results obtained for F and Cl show a much better agreement with the experimental data.

Calculations have been carried out on the low-lying excited states of ethylene, with a view of determining whether there appears a pyramidal arrangement of the bonds around each carbon atom, in addition to the twisting of the two CH2 groups. The two ~r-electrons and the two electrons of the CC c~-bond are treated as valence electrons; the core is formed by four hybrids pointing toward the hydrogen atoms, whose effect is neglected. The interaction between the valence electrons and the core is taken explicitly into account, but variation of the core energy with the distortions is neglected. It is found that in the (twisted) T state a flapping of the two CH~ groups produces a significant stabilization (0'7 ev for a tetrahedral arrangement of the bonds), whereas in the V state it causes an increase in energy. It appears from the study of the flapping in the non-twisted molecule that the relative stabilities of the cis-and trans-conformations in the ground state are not predicted correctly. This lack of success, which has been encountered also with other approximations in similar problems, stresses the need for more accurate calculations.

2014 Nous explorons l'analogie entre une nouvelle formulation du problème de la susceptibilité locale dans les alliages concentrés et la théorie de la localisation des états électroniques dans les systèmes désordonnés. La discussion ouvre la voie à une meilleure description et une meilleure compréhension de l'effet des propriétés magnétiques locales dans les systèmes désordonnés. En particulier, on considère l'effet des interactions entre amas et la nature de la transition de phase magnétique-non magnétique dans les alliages. Finalement, on discute la possibilité dans les alliages concentrés d'une généralisation de la notion de verre de spin. Abstract. 2014 We explore the analogy between a new formulation of the problem of local susceptibility in concentrated alloys and the theory of localization of electronic states in disordered systems. This discussion opens the way to a better description and understanding of the effect of local magnetic properties in disordered systems. In particular, the effect of cluster-cluster interactions and the nature of the magnetic-non magnetic phase transition in alloys is considered. Finally, we discuss the possibility in concentrated alloys of a cluster generalization of the spin glass phase.