I. The effect of the exchange‐only gradient correction
Previous work by the author on diatomic molecules and by others on polyatomic systems has revealed that Kohn–Sham density‐functional theory with ‘‘gradient corrected’’ exchange‐correlation approximations gives remarkably good molecular bond and atomization energies. In the present communication, we report the results of an extensive survey of density‐functional atomization energies on the 55 molecules of the Gaussian‐1 thermochemical data base of Pople and co‐workers [J. Chem. Phys. 90, 5622 (1989); 93, 2537 (1990)]. These calculations have been performed by the fully numerical molecules (NUMOL) program of Becke and Dickson [J. Chem. Phys. 92, 3610 (1990)] and are therefore free of basis‐set uncertainties. We find an average absolute error in the total atomization energies of our 55 test molecules of 3.7 kcal/mol, compared to 1.6 kcal/mol for the Gaussian‐1 procedure and 1.2 kcal/mol for Gaussian‐2.
II. The effect of the Perdew–Wang generalized‐gradient correlation correction
In an earlier paper [A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], Kohn–Sham density‐functional calculations of the total atomization energies of the 55 molecules of the Gaussian‐1 database of Pople and co‐workers [J. Chem. Phys. 90, 5622 (1989); 93, 2537 (1990)] were reported. We found that the local‐spin‐density exchange‐correlation approximation with a ‘‘gradient correction’’ for exchange gave an average deviation from experiment of only 3.7 kcal/mol. In the present work we assess the role of gradient corrections for dynamical correlation, and we enlarge our earlier survey to include 42 atomic and molecular ionization potentials and 8 proton affinities as well. We conclude that gradient corrections for correlation do not improve atomization energies, but are vitally important in electron nonconserving processes such as ionization.
0 Comments