Evolution of One-Particle and Double-Occupied Green Functions for the   Hubbard Model at Half-Filling With Lifetime Effects Within The Moment   Approach
  We evaluate the one-particle and double-occupied Green functions for the Hubbard model at half-filling using the moment approach of Nolting. Our starting point is a self-energy, $\Sigma(\vec{k},\omega)$, which has a single pole, $\Omega(\vec{k})$, with {\it spectral} weight, $\alpha(\vec{k})$, and quasi-particle lifetime, $\gamma(\vec{k})$. In our approach, $\Sigma(\vec{k},\omega)$ becomes the central feature of the many-body problem and due to three unkown $\vec{k}$-parameters we have to satisfy only the first three sum rules instead of four as in the canonical formulation of Nolting. This self-energy choice forces our system to be a non-Fermi liquid for any value of the interaction, since it does not vanish at zero frequency. The one-particle Green function, $G(\vec{k},\omega)$, shows the finger-print of a strongly correlated system, i.e., a double peak structure in the one-particle spectral density, $A(\vec{k},\omega)$, vs $\omega$ for intermediate values of the interaction. Close to the Mott Insulator-Transition, $A(\vec{k},\omega)$, becomes a wide single peak, signaling the absence of quasi-particles. 
S. Schafroth and J.J. Rodr\'iguez-N\'u\~nez