[Pw_forum] Fwd: phonon calculation

[Lorenzo Paulatto]

On 21/09/17 07:54, Suchitra Kumari wrote:
> I am looking for phonon calculation for silicon nanowire using QE6.0. 
> Because of the complex structure and larger number of atoms (64), the 
> calculation is taking too much time and not converging till the end.
> Will there be any extra parameters are to be incorporated for phonon 
> computations ?


Dear Suchitra,

1. Do you manage to converge at least one q-point, or not at all? 
Because the first q-point Gamma=(0,0,0) is usually faster than the 
following ones (if you have any symmetry). If you do not manage to 
complete at least Gamma, the other ones will be worst.

2. If you do manage to complete one point, it is better to run them 
one-by-one, using start_q and last_q.

3. Do you really need to save the rotated charge density 
(drho_star%open=.true.)? This part can take some time and a lot of 
memory, but it is only used by third order calculation. A d3 calculation 
is prohibitive with 64 atoms anyway, in any case it is much longer than 
the phonon calculation.

4. Do you really need 8 q-points (I guess your nanotube is oriented 
along z)? You are assuming that there is significant ion-ion interaction 
up to 8 unit cells away. An 8x8x1 grid is largely sufficient for 
graphene, which only has a much smaller unit cell, with just 2 atoms.



-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC -- CNRS & Université Paris 6
phone: +33 (0)1 442 79822 / skype: paulatz
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