Spin polarization and annihilation for radicals and diradicals

The extent of spin polarization in unrestricted Hartree-Fock and density functional B3LYP calculations has been determined for a few monoradicals as well as the corresponding diradicals containing these monoradicals as functional groups. Simple relations are found that explain the very high value for the average spin after projecting out the triplet component from a broken-spin calculation on the singlet state. The implications of these results for estimating the energy of the low-spin diradical by spin projection or by the Heisenberg Hamiltonian are discussed. (c) 2005 Wiley Periodicals, Inc.