Whereas the t - J model has gained wide popularity among physicists as a model for high-TC superconductivity, its physical origin is not at all clear. In this communication we show that the Hubbard model with occupation-dependent hopping (t1 - t3 - U model), recently proposed to account for the relaxation of doubly-occupied orbitals, reduces, in a physically relevant parameter region, to an effective t - J - t′ model with no upper bound on the J/t ratio. Results of exact diagonalization studies on finite size systems demonstrate the equivalence between the t1 - t2 - t3 - U model with intermediate/large U and the t - J - t′ even for very large J/t ratio (up to 16). On the contrary, t - U and t - J models turn out to be equivalent to t - J - t′ model only for very small J values (J much smaller than t). Implications of the results on high-TC superconductivity are briefly discusses.
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