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 (Jab) 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 ¦Jab¦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 Jab value for (CH2)2 is several times larger in the region R > 3.2 A 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.