Excited eigenstates and strength functions for isolated systems of interacting particles

Eigenstates in finite systems such as heavy nuclei and atoms, atomic clusters and quantum dots with few excited particles are known to be chaotic superposition of shell model basis states. Here we develop a method for description of this kind of eigenstates (ES) as well as of strength functions (SF). Using the model of n randomly interacting particles distributed over m orbitals we show that the average form of ES and SF in energy representation is given by the Breit-Wigner formula with the width Gamma which has a Gaussian dependence on energy. This explains evolution of ES and SF from the Breit-Wigner form for weak interaction to Gaussian form for strong interaction.