[Pw_forum] phonon accuracy and ecutrho for ultrasoft pseudopotential

[Nicola Marzari]

The energy is variational with respect to wfcs - so a first order error
on wfcs becomes just a second-order error on the energy.

For everything else you need tighter convergence on self-consistency.

In addition, higher plane wave cutoffs can help make sure that
e.g. the energy of the system doesn't change when translated,
thus recovering the zero phonons at gamma.

for the cutoffs, they are all studied and detailed in the SSSP page, 
mentioned many times.

				nicola

On 30/07/2017 17:03, balabi wrote:
> Dear developers,
>      I read this tutorial link 
> http://www.fisica.uniud.it/~giannozz/QE-Tutorial/tutorial_disp.html
>      It mentioned that if we use ultrasoft pseudopotential, we have to 
> use much higher ecutrho and tighter th2_ph
>      I have a few questions:
>      1. What is the reason for a higher ecutrho as well as th2_ph? Any 
> references?
>      2. Do we need a high ecutrho for PAW as well?
>      2. I tried to calc diamond phonon dispersion and use (4 4 4) grid 
> with C.pz-rrkjus.UPF. But I found that the phonon dispersions for 
> ecutrho=150 and ecutrho=450 are exactly the same. Then I don't 
> understand why the tutorial suggest such a high ecutrho as 450? Are 
> there other criterions for a suitable ecutrho that I missed?
>      3. For an arbitrary ultrasoft pseudopotential, how to quickly 
> determine a suitable ecutrho?
> 
> best regards
> 
> 
> 
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-- 
----------------------------------------------------------------------
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