[Pw_forum] phonon accuracy and ecutrho for ultrasoft pseudopotential

[Nicola Marzari]

Dear Balabi,


great that you did these tests - it's exactly the way to operate
if one wants to understand how things work.

> 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.

Well, not sure if the tutorial referred to the same pseudopotential, but 
you should indeed trust your calculations.
> 
>      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?

when you do a matdyn calculation on the full 4x4x4 mesh you also impose
the acoustic sum rules on its fourier transform (i.e. on the real
space constants) - things like making sure that the interatomic force
constants are such that a translation leaves the energy in the
quadratic hamiltonian exactly unchanged.

when you do a single point ph.x calculation you cannot impose these
symmetries/sum rules

so it's all consistent - enforcing an exact condition makes your 
convergence faster (to be honest, I do not have a proof for this -
but to give you an extreme example, if you had looked at the
3 lowest phonons at Gamma, those will always come out to be zero
from a mesh calculation with acoustic sum rule imposed (as they 
should)), but they will typically take very high cutoffs to converge
in a single point calculations (and often they do not even converge
exactly to zero, due to numerical noise in the xc functional (I think)).

			nicola


----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, EPFL
http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project