[DFTB-Plus-User] About Klines

[Jan Knaup]

Hello Giacomo and Nava,

It seems to me, you are referring to two different use cases. It is
correct, that to calculate the right electronic state, one needs to choose
a uniform sampling of the Brilluoin zone and that the exact choice of grid
depends on the crystal structure and the size of the model.

The k-lines mode however, is for creating band structure plots. Again, the
choice of the high symmetry points of interest depends on the crystal
structure and on convention. In many cases you will be comparing to results
in the literature and there you should simply choose the same points as
your reference.

The number of k-points along the line from one high symmetry point to the
next should be approximately proportional to the distance  between the
points. The total number of points is arbitrary and you should sinply
choose it to give you a smooth plot. 100ish points in total is usually a
good choice for a publication quality graph.

Best regards,

Jan Knaup

giacomo buccella <gek.buccella at gmail.com> schrieb am Di., 17. Jan. 2023,
14:57:

> Dear Nava,
> I suggest to write your questions to the mailing list of dftb+ users or to
> post it on the github community.
>
> However, as I can see from the manual, Klines is just a way to set your
> preferred k-point path. If you don't like it or it does not suits to your
> system, other methods are available. The manual is quite exhaustive on how
> is the correct syntax and how these methods are implemented. Please check
> to have the most recent version of the code and the relative manual.
>
> Apart from this, I'm not sure I understand what you mean.
> The first line is the first k-point, i.e. the starting point for the
> K-point sampling. In the next lines you specify how many points you want
> and towards which direction.
> I don't know why and how these examples have been conceived as they are.
> As far as I know, one should choose the k-point grid to be uniform in the
> BZ, and dense enough to get accurate results combined with reasonable
> computational costs. So I guess that, to construct the grid properly, one
> should have an idea of how the atoms dispose themselves in the unit cell.
>
> Hopefully, the community will answer more exhaustively.
> Best regards
>
> Giacomo
>
>
> Il giorno ven 13 gen 2023 alle ore 04:35 Nav Thakur <navthank147 at gmail.com>
> ha scritto:
>
>> Respected Giacomo Buccella,
>> Recently I have seen your mail in the DFTB+ community and wanted to ask
>> some questions. If you have time please help me through this.
>> I am new to the computational field. I don't have a basic understanding
>> of any of the software packages. I want to start with DFTB+ and I have just
>> started from scratch and facing difficulties with certain topics.
>>
>> These questions might be obvious and fundamental but I want to learn
>> without any misunderstanding. I am reading through the recipes listed in
>> the manual and on the website. The Kpoints used for the case given left me
>> perplexed. Aside from that, KLines'  confused me the most. I've seen videos
>> of QM and Siesta as well as WIEN2k, but these band lines were created as a
>> result of the Kpoints being established. However, DFTB+ caused
>> misunderstanding. I've been following the instructions in the handbook and
>> tutorials, so I don't know what I'm missing.
>>
>> For example, the manual shows,
>>
>> KPointsAndWeights = KLines{
>> 1     0.5   0.0   0.0
>> 10    0.0   0.0   0.0
>> 10     0.5   0.5   0.5
>> 1       0.0    0.0   0.0
>> 10     0.5    0.5    0.0
>> }
>>
>> *How are these lines 1, 10, 10, 1, 10 selected in these Klines? *
>> I went through the given tutorial and I didn't get how these high
>> symmetries points are selected. Please help through this.
>>
>> In the given tutorial of TiO2,
>> https://dftbplus-recipes.readthedocs.io/en/latest/basics/bandstruct.html,
>> it has following Klines
>>   # ...
>>
>>   KPointsAndWeights = Klines {
>>     1   0.5   0.5  -0.5    # Z
>>    20   0.0   0.0   0.0    # G
>>    45   0.0   0.0   0.5    # X
>>    10   0.25  0.25  0.25   # P
>>   }
>> }
>>
>> # ...
>>
>> *I don't know how these 1, 20, 45, and 10 are selected. How can I just
>> signify these lines Z, G, X, and P are in the 1, 21, 66, and 76
>> respectively?* Can't I signify others? Moreover, as I went through the
>> paper, they have different higher symmetry points than Z, G, X, and P and
>> the graphs of the band and DOS structure are also quite different. Can you
>> please help in this regard?
>>
>> I really appreciate any help you can provide.
>>
>> Thank you for your time.
>>
>> Sincerely,
>>  Nava Thakur
>>
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