[QE-users] phonon convergence problem of supercells

[Christoph Wolf]

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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