Improvement of the hybrid density functional method from the viewpoint of effective exchange integrals

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.