[Wannier] numerical issues with atoms at the center of the unit cell

[Aron Szabo]

Dear Jonathan,

setting the guiding_centres indeed solved the issue without shifting the 
atom coordinates.

Thanks,
Aron

On 08/16/2015 10:34 AM, Jonathan Yates wrote:
> Aron,
>
>   If you did not already, set guiding_centres=T in your wannier input file.
>   That might resolve the issue with the large spreads. If not report back and we’ll think some more.
>
> Jonathan
>
>
> On 14 Aug 2015, at 14:09, Aron Szabo <szaboa at iis.ee.ethz.ch> wrote:
>
>> Dear Wannier90 users and developers,
>>
>> I have experienced a strange behavior of Wannier90. I was using VASP to do the SCF simulation and generate the initial projections of a system, where I had scandium over MoS2. I was using Sc:sp3-1;Sc:l=2;Mo:l=2;S:l=1; as projections. And when running wannier90, most of the Wannier functions had reasonably small (2-3 A^2) spreads even at the first step, except those that were corresponding to the l=2 (d orbital) projections of the 3rd and 9th Sc atoms (>100 A^2), and they remained large during the minimization. It was suspicious that those atoms had direct z coordinates very close to 0.5, and I thought this error might have been related to this fact. So I have shifted all the atoms with 0.025 in the z direction, and calculated everything again, and now the spreads were as small for those orbitals as for every other, while the system itself was in fact the same, since a shift does not change anything in a periodic system. So it seems like there is some numerical instability when atom coordinates are very close to the half of a lattice vector. I don't know if it is related to VASP or wannier90. I can circumvent it easily now that I know it. I was just wondering if anyone else has experienced the same, or maybe have any guess where this error could originate from. I can provide additional details if anyone is interested. The original POSCAR file is below.
>>
>> Best regards,
>> Aron Szabo
>>
>> POSCAR:
>> MoS2+Sc
>>    3.18300000000000
>>      1.0   0.0    0.0
>>      0.0   1.73205080757   0.0
>>      0.0   0.0    9.5617407071622829
>>    Sc   Mo   S
>>      12     2     4
>> Direct
>>   0.0000000000000000 -0.0000000000000000  0.3265600783327190
>>   0.0  0.3333332999999996  0.4146745116137508
>>   0.0000000000000000 -0.0000000000000000  0.5006405198480147
>>   0.0  0.3333332999999996  0.5867734775290360
>>   0.0000000000000000 -0.0000000000000000  0.6736668439773074
>>   0.0  0.3333332999999996  0.7587831003021329
>>   0.5000000000000000 0.5000000000000000  0.3265600783327190
>>   0.5  0.8333332999999996  0.4146745116137508
>>   0.5  0.5000000000000000  0.5006405198480147
>>   0.5  0.8333332999999996  0.5867734775290360
>>   0.5  0.5000000000000000  0.6736668439773074
>>   0.5  0.8333332999999996  0.7587831003021329
>> -0.0000000000000000 -0.0000000000000000  0.2132268304255010
>>   0.5000000000000000  0.5000000000000000  0.2132268304255010
>>   0.0  0.3333332999999996  0.2689120454016644
>>   0.0  0.3333332999999996  0.1626395725698760
>>   0.5  0.8333332999999996  0.2689120454016644
>>   0.5  0.8333332999999996  0.1626395725698760
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>