Essentials of Computational Chemistry

8.6 GENERAL PERFORMANCE OVERVIEW OF DFT 285
spin-orbit corrections and updated experimental data for the heats of formation of the silicon
and beryllium atoms and carbonyl difluoride, while others do not. Readers may refer to the
original literature listed in the bibliography for full details.
Some key points may be inferred from Table 8.1.
1. For a given average level of accuracy, hybrid and meta-GGA DFT methods are obviously
the most efficient, showing mean unsigned errors almost equal in quality to the much
more expensive multilevel correlated methods. However, the maximum absolute errors
are larger with the former methods than the latter, indicating a slightly lower generality
even in the most current generation of functionals.
2. Hybrid and meta-GGA DFT functionals usually offer some improvement over corresponding
pure DFT functionals.
3. Increasing basis-set size does not always improve the accuracy of the DFT models,
although it must, of course, ultimately lead to a converged prediction.
4. Of the currently available DFT models, it is clear that GGA models offer a major
improvement over the older LSDA model. It further appears that the P86 functional
should be avoided. With respect to a more specific ranking of functionals, Boese, Martin
and Handy (2003) have carried out a careful evaluation of a large number of functionals
for various energetic quantities (atomization enthalpies, IPs, EAs, geometries, etc.) using
polarized triple-ζ basis sets. They conclude that for pure GGA functionals an error
ranking (i.e., lower is better) is HCTH < OLYP < BPW91 < BLYP