SIMPLE SPIN CORRECTION OF UNRESTRICTED DENSITY-FUNCTIONAL CALCULATION

The unrestricted Hartree-Fock and unrestricted Kohn-Sham calculations generally result in spin-contaminated solutions. Moreover, the energie s from these calculations cannot be directly compared with the results of corresponding restricted calculations since the latter yield highe r energies due to restrictions imposed on the form of the wave functio n. We present here a simple method of correcting the mixed spin energi es resulting from unrestricted Hartree-Fock or density-functional theo ry calculations and removing the foreign spin components. The method a llows for elimination of higher-multiplet components from the given mi xed spin state solution by performing unrestricted calculations at the same fixed geometry for the higher multiplets and the state under con sideration. The performance of the method is illustrated with several examples of density-functional calculations of radical species. The cu rrent method is also variational in nature and can be further extended in a self-consistent field fashion.