[SPAM] Troubles calculating Z2 (graphene)

[Dominik Gresch]

EDIT: It should be lambda t1, t2: [t1 / 2, t2, 0]. The second parameter 
should always go across the full BZ, or back into the same point.


On 21.11.2016 17:32, Dominik Gresch wrote:
>
> Hi Jimena,
>
> First of all, I think your problem might just be this: The surface 
> should be lambda t1, t2: [t1, t2 / 2, 0] not lambda t1, t2: [t1, t2, 
> 0]. This is because the Z2 invariant is calculated between the two 
> time-reversal invariant lines, not across the full BZ.
>
> Here are a few other remarks:
>
> As Liucus noted, a problem arises when the direct band gap is very 
> small. In principle the Z2 invariant can be calculated whenever the 
> direct band gap is not zero anywhere on the surface you are 
> calculating. However, the default values for the convergence 
> parameters are better suited for common "well-behaved" systems. When 
> the band gap is very small at some point, the Wannier charge centers 
> can move very quickly. As a consequence, you should use a large number 
> of starting lines (you've done that already), and 'min_neighbour_dist' 
> should usually be rather small (this is just the smallest possible 
> distance between calculated Wannier charge center lines before the 
> iteration stops).
>
> Note that the 'gap_tol' doesn't check for the band gap -- the gap in 
> question here is the gap in the Wannier charge center spectrum (the 
> check makes sure the middle of the gap is not too close to a 
> neighbouring WCC). Z2Pack doesn't have knowledge of the band gap 
> itself, but it just so happens that systems where the band gap is 
> small are also troublesome for the 'gap_tol' check.
>
> I tried to run your input files, but it seems the nscf and pw2wan 
> input files are missing. If the problem persists, please send me these 
> so I can try to solve the problem.
>
> Best regards,
>
> Dominik
>
>
> On 21.11.2016 01:52, liubi at mail.ustc.edu.cn wrote:
>> Hello Asar,
>>     I don't think graphene is a good example since the graphene is 
>> nearly gapless even with SOC. The gap check in Z2PACK will always fail.
>>
>> Liucus
>>
>> 在2016-11-18 19:49:37，Jimena Olmos Asarjimenaolmos at gmail.com 
>> <mailto:jimenaolmos at gmail.com>写道：
>>
>>     Good morning,
>>     I am trying to learn how to run the Z2Pack, since it would be
>>     really useful for my research project. After some trials, I could
>>     get the expected results for two examples you offer: Bi and
>>     Bi2Se3. Now I would like to test some known system, like
>>     graphene, to check if what I'm doing is correct. Well, it may not
>>     be the case, since I can't get Z2=1. Would you, please, give me a
>>     hint? I attach the "run.py" file, scf and wannier90 inputs. I
>>     also add the pseudopotential (it was generated using the code by
>>     Andrea del Corso, within the QE package).
>>     Thanks in advance,
>>     Jimena Olmos Asar
>>
>

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