2 J un 1 99 7 Non - rigid Shell Model and Correlational Mechanism of the Local Pairing

The Hartree-Fock states of the many-electron atomic system can be unstable with respect to a static or dynamic shift of the electron shells. An appropriate non-rigid shell model for atomic clusters is developed. It permits to formulate a convenient approach to the semiempirical description of the different correlation effects and to reveal some new effects. In the framework of conventional Hartree-Fock (HF) scheme the description of some specific correlation effects in atomic systems requires too large set of HF-configurations. This is connected, in particular, with the high density of excited states, with possible anomalously strong influence of small perturbations, and the instability with regard to a formation of strongly correlated states. In such situations, as a rule, special model approaches are used. One of the most simplest descriptions of the correlation effects is connected with the taking into account shifts of atomic orbitals of the conventional MO-LCAO-scheme [1]. For doubly occupied orbitals (biorbitals) with shifted electron shells