[DFTB-Plus-User] Si20 FPLO-geometry does not SCC-converge

Jan Knaup janknaup at gmail.com
Mon Jun 2 12:18:13 CEST 2014


Dear Vivien,

the clusters you show are highly symmetric and have dangling bonds. It is
extremely likely that there are many degenerate states around the HOMO-LUMO
gap, in which case occupation can oscillate strongly.
Try setting the temperature of the Fermi Function to a finite value, you
coul dstart with 10K and if that does not work, try 300K. In both cases,
the effect on total energies etc will be very small. Once you have a
converged geometry with finite temperatutre, you can try again with 0K.
(I am not sure, if Fermi filling with 0.0K can even work, if you don't want
a Fermi distribution, just comment out the line. DFTb+ will then fill the
states according to Aufbau principoe).

Best,

Jan

Jan M. Knaup                      | Fon +49-(0)421-218-62351
Dipl. Phys. Dr. rer. nat.         | Fax +49-(0)421-218-62770
Universität Bremen - BCCMS        |
Am Fallturm 1                     | Jan.Knaup at bccms.uni-bremen.de
28359 Bremen                      | JanKnaup at gmail.com
Germany                           | www.bccms.uni-bremen.de


2014-06-02 11:58 GMT+02:00 Vivien Lorenz <v.lorenz at ifw-dresden.de>:

> Dear all,
>
> I've been trying to get a certain Si20 cluster converged with DFTB - a
> 3-fold symmetric structure as shown on the webpage
> http://altair.physics.ncsu.edu/projects/sismall/siclust.html. I did not
> have the coordinates of the structure, so guessed at them and firstly
> optimized the geometry of the cluster under the given symmetry constraints
> with the DFT code FPLO - no problems there. Now I used the resulting atomic
> position as an input to DFTB, but I do not get the charges
> self-consistently converged.
> I have checked all coordinates - they are given in the same units in the
> FPLO as well as in the DFTB output, and they are identical. All other DFTB
> input files already worked for many other calculations. I am appending my
> input files.
> Do you have any suggestions for me?
>
> Thanks a lot,
> Vivien
>
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