Diradicals and diradicaloids in natural orbital functional theory.

Natural orbital functional (NOF) theory in its PNOF3 and PNOF4 implementations is used to investigate the electronic structure and energetics of selected diradicals and diradicaloids: the trimethylenmethane (TMM) diradical and the imino-allyl (IMA) and oxyallyl (OXA) diradicaloids. These compounds are taken as paradigmatic cases of molecules with full and partial diradical character, and are considered as a case study to illustrate the potentiality of Piris NOF (PNOF) to yield the correct treatment of near-degeneracy effects in diradicals. The degree of diradical character is determined through the inspection of the corresponding natural orbital occupation numbers. In addition, the energetics of these compounds with respect to their cyclic isomers (methylenecyclopropane, MCP, iminocyclopropane, ICP, and cyclopropanone, OCP) is investigated. We have found that PNOF4 shows a promising description of these species, and yield the correct trends in occupation numbers as compared with wave-function methods such as CASSCF. Therefore, PNOF4 can give a correct description of near-degeneracy effects, and hence, is a promising method to treat diradicals and diradicaloids in chemistry, a delicate type of molecular systems to simulate by theoretical methods.