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

[Vivien Lorenz]

Dear Jan,

thanks for your advice - your suggestions with a finite filling 
temperature of 10 K (or more) worked very well. Filling according to the 
Aufbau principle did not work, but it was a nice piece of information, 
anyways.

Best,
Vivien

On 2014-06-02 12:18, Jan Knaup wrote:
> 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 [3]
> 
> 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 [1]. 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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> 
> 
> 
> Links:
> ------
> [1] http://altair.physics.ncsu.edu/projects/sismall/siclust.html
> [2] http://www.dftb-plus.info/mailman/listinfo/dftb-plus-user
> [3] http://www.bccms.uni-bremen.de
> 
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