Dear nicola,
Thank you so much for your explanation.
I made a further investigate.
I tried to calculate a series of diamond phonon dispersion respect to different ecutrho.
They are all calculated under the following condition
1. scf condition: kmesh=4,4,4 ecut=27, pseudopotential=C.pz-rrkjus.UPF, conv_thr = 1.0d-10
2. phonon condition: qmesh=4,4,4 tr2_ph=1.d-14
I plot these phonon dispersion together (here is the image https://pasteboard.co/GDEmkd3.png ) . We can see that except for ecutrho=75, dispersion for all other cases are quite close. The biggest deviation for ecutrho=100 to 450 is 8.2275cm^-1, while for ecutrho=150 to 450, the biggest deviation is almost as negligible as 0.7cm^-1. This is quite surprising，because the tutorial link recommended high ecutrho=450 for converged calculation.
Besides, I extract X point frequency from the above dispersions and plot them with respect to ecutrho ( here is the image https://pasteboard.co/GDEqdDb.png )
On the other hand, I can also calculate phonon frequency at X point (0,1,0) using ph.x directly with the same scf parameter as the above. Again plot them with respect to ecutrho (Here is the image https://pasteboard.co/GDDu3wt.png ).
From these two plots, I got confused. The convergence behaviour of matdyn.x and ph.x at the same point is not the same. The convergence of dispersion is much quicker ( above ecutrho=200) then convergence of single point ph.x calc ( above ecutrho=400 ). Is it generally true?
best regards
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