[QE-users] phonon convergence problem of supercells

[Christoph Wolf]

Dear Bhamu,

thank you for your email. My system is bulk CsPbBr3 and I do not have any
problem with SCF self-convergence. Just the phonon calculation. Changing
mixing_alpha did not lead to any significant improvement in my case....

Best,
Chris

On Wed, Jun 27, 2018 at 9:23 PM, Dr. K. C. Bhamu <kcbhamu85 at gmail.com>
wrote:

>
>
> Hii Christoph,
>
> Could you please confirm whether your system is homogeneous or not?
>
>
> If it is not a homogeneous then 'local T-F' scheme will be the best one.
>
> I recently solved my problem by using local TF.
>
> Also, you can reduce mixing parameter to 0.3 from 0.7
>
>
>
> Kind regards
> Bhamu
>
> Dr. K. C. Bhamu
> National Postdoctoral Fellow,
> Physical and Materials Chemistry Division
> CSIR-NCl, Pune
> India
> Mob. No.  +91-7888227869
>
> On Wed, Jun 27, 2018 at 5:46 PM, Christoph Wolf <
> wolf.christoph at qns.science> wrote:
>
>> Dear all,
>>
>> I have recently encountered an "interesting phenomenon".
>>
>> Let's say I have determined the (zone boundary phonon frequency)
>> convergence parameter for a unit cell to be a k-grid of 6x6x6 and a
>> ecutwfc=120 Ry. If I now take a 2x2x2 supercell of the material (using the
>> relaxed cell as reference) I (naively) assumed that that would translate to
>> a 3x3x3 k-grid, however I can never manage to converge phonons with that
>> kind of setting as during the initial electric fields calculation the
>> minimization will start to oscillate like this:
>>
>>
>>       iter #  17 total cpu time :  8249.5 secs   av.it.:   1.8
>>       thresh= 2.804E-05 alpha_mix =  0.700 |ddv_scf|^2 =  1.396E-07
>>
>>       iter #  18 total cpu time :  8353.6 secs   av.it.:   1.0
>>       thresh= 3.737E-05 alpha_mix =  0.700 |ddv_scf|^2 =  1.506E-08
>>
>>       iter #  19 total cpu time :  8876.9 secs   av.it.:  15.4
>>       thresh= 1.227E-05 alpha_mix =  0.700 |ddv_scf|^2 =  6.969E-08
>>
>>       iter #  20 total cpu time :  9303.8 secs   av.it.:  12.1
>>       thresh= 2.640E-05 alpha_mix =  0.700 |ddv_scf|^2 =  6.076E-08
>>
>>       iter #  21 total cpu time :  9751.4 secs   av.it.:  12.9
>>       thresh= 2.465E-05 alpha_mix =  0.700 |ddv_scf|^2 =  3.246E-08
>>
>>       iter #  22 total cpu time : 10145.8 secs   av.it.:  11.1
>>       thresh= 1.802E-05 alpha_mix =  0.700 |ddv_scf|^2 =  3.519E-08
>>
>>       iter #  23 total cpu time : 10542.9 secs   av.it.:  11.2
>>       thresh= 1.876E-05 alpha_mix =  0.700 |ddv_scf|^2 =  3.168E-09
>>
>>
>> Is this a common problem or am I doing something completely wrong...?
>>
>> Your help is very much appreciated!
>>
>> Best,
>> Chris
>>
>> --
>> Postdoctoral Researcher
>> Center for Quantum Nanoscience, Institute for Basic Science
>> Ewha Womans University, Seoul, South Korea
>>
>>
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>
>


-- 
Postdoctoral Researcher
Center for Quantum Nanoscience, Institute for Basic Science
Ewha Womans University, Seoul, South Korea
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