Non - rigid shell model and novel correlational effects in atomic and molecular systems

Direct analytical and numerical calculation show that two-electron atomic configuration can be unstable with respect to a static or dynamic shift of the electron shells. This enables to develop a so called non-rigid shell model for a partial account of the electron correlations within atomic clusters in solids. In a framework of this model a correlated state of two-electron molecular configuration is described by a set of symmetrized shell shifts q γ similarly to the well known shell model developed for a description of the lattice dynamics. A set of q γ-shifts are found after minimization of the energy functional. We present a number of the novel unconventional effects including: i) a correlational mechanism of the local pairing; ii) a correlational (pseudo) Jahn-Teller effect provided by a joint account of the electron shell shifts and conventional nuclear displacements; iii) an appearance of the chiral correla-tional states. The model allows an introduction of the pseudo-spin formalism and effective " spin-Hamiltonian " for a description of the short-and long-range ordering of non-rigid atomic backgrounds in crystals. Finally, the model can be readily built in the conventional band schemes. 1 Introduction Electronic correlations is one of the fundamental problem in a theory of atoms, molecules and solid state, particularly for the systems with high density of excited states when a small perturbation can result both in drastic reconstruction of the energy spectrum and in modification of the ground state up to formation of a strongly correlated state. As a rule, in such a situation an appropriate description of the ground state within the bare restricted basis often requires a lot of configurations or considerable extention of the basis, and so becomes difficult for practical realization and interpretation. Namely this situation occurs in atoms, where description of some specific correlation effects in terms of Hartree-Fock basis requires a large number of Hartree-Fock configurations. Such a problem implies a search for alternative variational approaches to the electronic structure and energy spectrum. In this work we develop further the variational method for the many-electron atomic clusters with the trial parameters being the coordinates of the center of the one-particle atomic orbital [1]. The resulting shift of the atomic orbital allows to interpret the variation of the electronic density distribution rather clearly, and the symmetry of a system can be readily used for construction of the trial many-electron wave function. The shifted electronic shells in conventional MO-LCAO-scheme …