| The energy of cohesion for a narrow s band is computed using various approximations (Hubbard, CPA, Gutzwiller) developed to second order in the Coulomb correlation ratio U/w. The results are compared with that obtained by the perturbation method in the single site tight binding approximation, The two first terms (band and Hartree Fock) are identical in all these approximations. The next term in the development, due to correlations, gives slightly different results in the various approximations. A possible improvement is discussed. Computations are carried out for a rectangular and symmetrical band of width w, but are shown not to be sensitive to the form of the band. |
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