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emiliano

emiliano.burresi@enea.it

Dear Andrei

Thank you for your suggestions and sorry for delay.

I’ve already tried to use the Hartree-Fock scheme (I used fshift,SOSCF,DIIS,HONDO,...) to prepare a better guess in order to run a GGA calculation,

but unfortunately neither HF converges.

Now I’m trying to change the basis set,using a larger basis set to improve the description.

Kind regards, emiliano

On Fri Mar 23 '12 1:09am, Andrei V Scherbinin wrote

---------------------------------------------------

>Dear Emiliano,

>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

Tue Mar 27 '12 3:33pm

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