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^\gamma similarly to the well known shell model developed for a description of the lattice dynamics. A set of q^\gamma-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 correlational 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. 
Yu.D. Panov, A.S. Moskvin