Density-functional thermochemistry. III. The role of exact exchange

Despite the remarkable thermochemical accuracy of Kohn-Sham density-functional theories with gradient corrections for exchange-correlation, we believe that further improvements are unlikely unless exact-exchange information is considered. A semiempirical exchange-correlation functional containing local-spin-density, gradient, and exact-exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first- and second-row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.