A study of the 2D Hubbard model in the paramagnetic phase by means of the COM is reported in Refs. [37,2,38,39,40,41,3,4,42,43,44,45,42]. In particular the following properties have been computed:
The COM results have been compared with the ones obtained by:
The paramagnetic solution of the two-dimensional Hubbard model has been widely studied within the COM; it would require too much space to summarize all works. Among the most relevant results we note the good agreement with the data of numerical simulation for all local, integrated and thermodynamical quantities (chemical potential, double occupancy, local magnetic moment, internal energy, specific heat). It is also worth mentioning the calculation of the response functions by means of the Hubbard propagator, in the one-loop approximation, which gives good agreement with the quantum Monte Carlo results. The single-particle properties have been also studied by considering an approximation where the two-site correlations are combined with the resolvent method. The lower and upper Hubbard subbands have a three peak structure at half filling. By doping a coherent peak develops at the Fermi level. The scenario well reproduces the results of numerical simulation.