DFT is rapidly devolving into just another semi-empirical method whose results are so wildly variable as a result of choice of functional as to be (at a minimum) viewed with skepticism or (at worst) dismissed as totally unreliable.
You'd probably be better off using the improved MP2 or MCSCF methods incorporated into Firefly. At least they are well defined, their limitations well established, and their results are not subject to the whim of critical parameters being fit to pseudo-randomly selected empirical and/or theoretical "reference" systems.
On Mon Jun 21 '10 6:11pm, Miro Moman wrote
>I am modelling an iron-sulfur cluster. To the best of my knowledge, Szilagyi and Winslow (J. Comput. Chem. 2006, 27, 1385-1397) have carried out the most systematic optimisation of DFT functionals for this kind of systems to date.
>They recommend using a hybrid functional with 5% HF exchange and 95% density functional exchange supplemented with Perdew's 1986 correlation functional. Can I implement this in Firefly? Which keywords should I use?
>If defining custom functionals is not possible, which of the available functionals would you recommend?