[QE-users] how to choose properly the parameters value of nq1, nq2 nq3 in the ph.x inputfile.

[Gang Yang]

Dear QE experts: 
I am doing phonon dynamics calculation. I want to obtain the second force constants using QE ph.x and q2r.x module. For the ph.x calculation, how to choose properly the parameters value of nq1,nq2 nq3 in the ph.x inputfiles. As you know, the parameters nq1,nq2,nq3 determine the number of outputfile dyn. I saw someone using nq1=2,nq2=2, nq3=2. or nq1=8,nq2=8,nq3=8. et al . if the paramter nq1,nq2 nq3 is too large, the calculation time is too long. if the parermeters nq1,nq2, nq3 is too small, I don't konw whether it will affect the precision of second force constants. in addtion, i also found the some specical nq1,nq2,nq3 value are not allowed for asymmetric structure. therefore, my question is if I want to obtain a reasonable second force constants using q2r.x, how I can choose the parameters value of nq1,nq2 nq3 when I using ph.x calculation in 10 atom primitive cell? 
Thank you very much. 
have a good day. 

Yang Gang 
Institut des Nanotechnologies de Lyon (INL) - UMR5270 
Ecole Centrale de Lyon 
36, Avenue Guy de Collongue 
69 134 Ecully Cedex 
Tél.: +33 (0)7 64 83 01 70 

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