[Pw_forum] ph.x run on GRID including drho_star for d3q.x calculation

[Lorenzo Paulatto]

On Sunday, March 12, 2017 11:00:58 PM CET Min Hong wrote:
> Hi Lorenzo,
> 
> Thanks for your kind response.
> 
> Is it enough to calculate the phonon linewidth and thermal conductivity just
> using the Gamma point?

Well, I cannot know. What I can tell is that the 2-body force constants (aka 
phonons) are probably longer range than the 3-body force constants (aka d3). 

You can do the phonon calculation on a fine grid (and only export fildrho at 
Gamma, use start_q, last_q)  and then do d3 only at Gamma: the linewidth 
machinery does not need the ph and d3 initial grids to be equal. 

If your cell is large enough, samplig just the gamma point for the 3-body 
force constans may be enough, also thanks to periodic boundary conditions, 
Fourier interpolation, you already get quite a bit of information.

Full disclosure: the performance advantage of perturbation theory, w.r.t a 
finite-difference calculation, is not evident at the Gamma point. You can still 
get better accuracy and you won't have to check the derivation w.r.t. the 
finite displacement, but you'll be limited to norm-conserving psp and non spin 
polarized calculations. So if you can only do the Gamma point anyway, it may 
be worth considering a finite difference calculation.

hth


-- 
Dr. Lorenzo Paulatto 
IdR @ IMPMC -- CNRS & Université Paris 6
phone: +33 (0)1 442 79822 / skype: paulatz
www:   http://www-int.impmc.upmc.fr/~paulatto/
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