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Andrei V Scherbinin
andrei.scherb@gmail.com

I would suppose that your system is far from trivial, and probably
your calculation suffers from narrow-bandgap problems due to rather
large size of the cluster. In addition, plain GGA DFT (BLYP in your
case), tends to underestimate bandgaps in materials.

I can not guarantee that the following advices help, but try the
following:

1) try to converge pure HF first (perhaps to a large treshold), then
start DFT SCF from converged HF orbitals. Usually, RHF performs
better because it tends to overestimate bandgaps as opposed to DFT.

2) Try diferent SCF convergence options (level shift, SOSCF vs DIIS, etc)
during both HF/DFT stages if problems still appear.

2) If RHF converges but pure DFT still fails to converge starting from
the RHF MOs on input, may be it is reasonable to perform a sequence of
hybrid DFT calculations with gradually decreasing HF exchange fraction
(say, HFLYP, then BHHLYP, B3LYP, BLYP). As a rule of thumb: the larger
the HF fraction, the better the convergence usually is - see above.

3) If all that doesn't work, re-consider your system again. May be,
the closed shell description fails, or some surface-localized states
emerge, or the minimal basis set is a poor choice (d-AOs are usually
rather important for 3rd row elements), etc...


Best wishes, Andrei




On Mon Mar 19 '12 4:10pm, eburresi wrote
----------------------------------------
>Dear firefly users

>I'm trying to perform DFT-BLYP calculation on CdS nanocluster with 48 Cd atoms and 48 S atoms, using 3-21g and sto-3g basis set,
>but I'm finding some convergence problems during SCF optimization. I tried to use combo,
>building an intial guess from  optimized molecular orbitals of sulphur and cadmium fragments separately but in my case didn’t help a lot.
>The atom coordinates are from CPMD ab initio molecular dinamics simulations.
>I hope that someone can help me.

> $CONTRL SCFTYP=RHF  RUNTYP=ENERGY NOSYM=1 COORD=UNIQUE UNITS=ANGS
> ICHARG=0  MAXIT=2000 DFTTYP=BLYP  $END
> $SYSTEM TIMLIM=600 MEMORY=70000000 $END
> $SCF    DIRSCF=.True. DIIS=.T. FDIFF=.F.
>          SHIFT=.T. SOSCF=.F.  $END
> $GUESS  GUESS=HUCKEL $END
> $BASIS  GBASIS=N21  NGAUSS=3  $END
> $DATA
>CdS Cluster 96 Temperature 340K
>C1
>
>
>Thank you in advance
>  Emiliano