Intersite Coulomb interaction and Heisenberg exchange

Based on exact diagonalization results for small clusters we discuss the effect of intersite Coulomb repulsion in Mott-Hubbard or charge transfers insulators. Whereas the exchange constant J for direct exchange is enhanced by intersite Coulomb interaction, that for superexchange is suppressed. The enhancement of J in the single-band models holds up to the critical value for the charge density wave (CDW) instability, thus opening the way for large values of J. Single-band Hubbard models with sufficiently strong intersite repulsion to be near a CDW instability thus may provide physical realizations of t-J-like models with the ''unphysical'' parameter ratio J/t approximate to 1.