We have optimized a heme with 2 coordinational amino acids (above and under the heme plane), it is a big chromophore molecule with a Fe atom in the center and containing more than 100 atoms. The heme in high-spin reduced state was used for calculations. We used LANL2-DZ external basis-ecp combination, d5 was turned on, maxit,icut,itol and inttype was set up to improove convergency. In the SCF group following parameters was set:
fdiff=.f. diis=.t. soscf=.f. dirscf=.t. damp=.t. shift=.t. nconv=6
All this settings allows us to procceed geometry optimization in delocolized coordinates as well. Every step takes about 40-70 iterations of SCF for convergency. But in some conformations convergency takes more than 200 steps and the question is how to improve with Firefly abilities.
In GAMESS-US (december 2011) convergency appears to be slightly better. May be because of step-by-step procedures: pure UHF -> UDFT in coarse grid -> UDFT in fine grid. I didn't find this ability in Firefly. May be firefly have some alternative techniques to improve the convergency in our case?
DLC coordinates for optimization works in Firefly better than in GAMESS-US so we tend to use Firefly-only for energy minimization if UDFT-convergency can be improoved.