Beschreibung:
An important method in many-particle physics is related to the Hubbard model which can be used to investigate the behavior of fermions, particularly to study exchange and correlation of electron- electron interaction. For a special cluster, the molecule consisting of two atoms, the electrons interact via one hopping integral, and for a small number of N = 1; 2; 3 electrons, the model has the advantage that it can be solved exactly. Therefore, some important physical quantities like the Green's function, the spectral function and the self-energy are calculated and discussed. Exchange and correlation are fully treated. The self-energies obtained by two approximate methods, namely the Hartree-Fock and the GW approximation, are compared with the exact self-energy. The comparison allows a discussion of the validity of the approximations versus the ratio of Hubbard parameter and hopping integral.