  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.
