[QE-users] ?==?utf-8?q? phonon convergence problem of supercells

[JAY Antoine]

Dear Christoph,

Try to decrease alph_mix to 0.1. Is it enough to converge?

Best regards,

Antoine Jay

On Wednesday, June 27, 2018 14:16 CEST, 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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