[CP2K-user] [CP2K:12933] Re: Issues with tight binding methods

[Pierre Cazade]

Hello Maxime,

Thank you for the detailed answer. I will have a look at DFTB+ software.

Regarding cp2k, if I understand well, it is better so far to only use 
the scc and non-scc parameters which are provided within the data 
section of the code and not the more advanced parameters found on DFTB 
website.

Regards,
Pierre

On 27/02/2020 09:12, Maxime Van den Bossche wrote:
> Hello again,
>
> I had a closer look at the 3ob parameter set 
> <http://www.dftb.org/parameters/download/3ob/3ob-3-1-cc/> and it seems 
> like not all the
> 'bells and whistles' have been implemented in the DFTB module in CP2K
> to make proper use of this parametrization:
>
> - It is supposed to be used with 'full' third-order terms, not just 
> the diagonal ones
> (note: there are also the LMAX_DFTB and DFTB3_PARAM keywords which
> need to be set under CP2K_INPUT/FORCE_EVAL/SUBSYS/KIND).
>
> - Becke-Johnson damping of the D3 correction doesn't seem available in the
> DFTB module (though it is available for e.g. DFT calculations).
>
> The simplest way to deal with this, I think, is to use the DFTB+ code 
> instead of
> CP2K. As an example, I have attached a ZIP file with DFTB+ input files 
> and the
> final geometry I got out of it. There is a +20% expansion in the 
> c-direction.
> I have no real experience with DFTB calculations of molecular 
> crystals, so I don't
> know if this is a typical deviation or not. Where did the initial 
> geometry come from?
>
> /Maxime
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-- 
Dr Pierre Cazade, PhD
AD3-023, Bernal Institute,
University of Limerick,
Plassey Park Road,
Castletroy, co. Limerick,
Ireland

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