[DFTB-Plus-User] : Simulating Silicon Crystal : Does DFTB estimate the phase of the wavefunction accurately?

[Ben Hourahine]

Hello Fahd,

I think Bálint answered everything already. But there is one slight
subtle point about the phase, which is that at each k-point there is a
global phase of the wave-function. Currently this is undefined between
different k-points (it will depend on your particular eigensolver and
its numerical implementation). If you are calculating properties at each
k-point separately then combining, then this is harmless, but if you
want to calculate between k-points this can cause some complications.

The global phases can be made consistent, but this is not available in
the current public code (its related to the Berry phase of the
wavefunction).

Regards

Ben

On 25/05/18 07:55, Bálint Aradi wrote:
> Dear Fahd,
>
>> (data from wp-1-184-5-real.cube) + 1i* (data from wp-1-184-5-imag.cube).
>>
>> Is the above combination a correct way for estimating the wave function,
>> and will the relative phase between real and imaginary parts be treated
>> correctly at all coordinates in space? 
> Yes, the real and imaginary cube files are built using the real and
> imaginary part of the complex eigenvectors, respectively. The way you
> combine them seems to be correct and you should get back the correct
> complex wave function everywhere. (Unless we have a bug in waveplot.)
>
>> I have also tried playing with different unit cells (primitive and
>> cubic) for silicon in DFTB. /The results from different unit cells give
>> me the same charge densities, but the Fourier transforms of the wave
>> functions are different. /Hence, I was wondering whether I am combining
>> the wave functions correctly, and does DFTB take into account the phase
>> of the wave function accurately. 
> Yes, I am pretty sure, DFTB+ takes the phase of the wave-function
> correctly into account.
>
> Just to make a check, you may consider to calculate a single atomic
> hydrogen chain (orthogonal supercell with two supercell vectors set to
> large values). That you could compare to results obtained by other means
> (e.g. either VASP calculation or even analytic ones) and spot, if there
> are any bugs.
>
>   Best regards,
>
>   Bálint
>
>
>
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      Dr. B. Hourahine, SUPA, Department of Physics,
    University of Strathclyde, John Anderson Building,
            107 Rottenrow, Glasgow G4 0NG, UK.
    +44 141 548 2325, benjamin.hourahine at strath.ac.uk

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