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Spin-unrestricted hybrid density functional theory (DFT) methods are improved to reproduce potential curves and effective exchange integrals ( J ab ) of several diradical models, which were calculated by unrestricted post-Hartree-Fock methods. Particularly, we attempt to reproduce the J ab value of the UCCSD(T) method by using the DFT methods. In general, the magnitude of the calculated J ab values by the conventional DFT methods for radical dimers was too large in comparison with the post-Hartree-Fock methods values. To overcome the problem, we improve the DFT method for the intermolecular (through-space) effective exchange integrals by using the hybrid method. Furthermore, the rule for the hybridization is constructed in terms of instability value y sys of the bonding. FIGURE 7. The difference of the effective exchange integrals J ab between LDA(SVWN) and GGA(BLYP) at the dissosiation region.dimers are very weak. The whole y sys of the Cr(II) dimer are overestimated.From this table, we summarize that J ab [APUCCSD(T)] correspond to 1. J ab (S2VWN; c i = 0.8) when y sys is about 80%. 2. J ab (S2VWN; c i = 0.5) when y sys is about 60%. 3. J ab (S2VWN; c i = 0.6-0.7), when y sys is about 70%.