EFFECTIVE EXCHANGE INTEGRALS FOR OPEN-SHELL SPECIES BY DENSITY-FUNCTIONAL METHODS

Unrestricted Kohn-Sham (UKS) density functional theories (DFT) are applied to the calculation of potential curves and effective exchange integrals (J(ab)) for open-shell species. It is found that approximate spin-projected UKS DFT reproduce experimental singlet-triplet gaps for monocentric diradicals such as O, NH and CH2, and indicate an exponential decay of \J(ab)\ with increasing intermolecular distance (R) in the dimer of triplet methylene, in conformity with the size-consistency condition. However, the potential curves for the singlet state for the dimer of triplet CH2 by UKS DFT are deeper than those of ab initio second-order CI and CASPT2(D) and, therefore, the J(ab) value for (CH2)2 is several times larger in the region R > 3.2 angstrom than the corresponding ab initio value. The implications of these results are discussed in relation to the necessity of the self-interaction correction to UKS DFT and the applicability of UKS DFT to molecular magnetism.