[Pw_forum] How to set ecutwfc and ecutrho

[Gabriele Sclauzero]

Dear Jiaye,

Jiaye Li wrote:
> Dear users
> 
> I am curious about the calculation speed related to "ecutwfc" and 
> "ecutrho". As indicated in the tutorial, ecutwfc represents the energy 
> cutoff (Ry) and ecutrho refers to kinetic energy cutoff for charge 
> density and potential. I cannot fully understand the meaning of that 

More precisely, ecutwfc and ecutrho are the kinetic energy cutoff of the plane wave basis 
used to describe the wavefunctions and the charge density, respectively.

> brief description. From my experience (vasp), the larger the energy 
> cutoff, the slower the calculation would be, but more accurate. I just 

That's right and I think should be a general consideration valid for any code using plane 
waves. I would say "longer", rather than "slower" (which I feel more appropriate with 
regard to the optimization of a code or the performance of a computer).


> would like to know how to set the experience values of the two flags for 
> the case of metal bulk (and surface) and metal oxide bulk (and surface)? 

The choice of the cutoffs depends only on the pseudopotentials you are using and on which 
quantities you are interested in (for instance, magnetic properties are harder to converge 
wrt total energy). Once you've chosen your PPs you can test convergence of the energy wrt 
to cutoffs in the cheapest system (usually in the bulk). The threshold of convergence is 
somehow your choice: personally I don't go below 1mRy since this is the expected order of 
magnitude of the errors due to PPs.
For normconserving PPs you don't need to specify ecutrho, since the default (4*ecutwfc) is 
good (you can try to figure out why, writing down the charge density in terms of the PW 
expansion of the wavefunctions).
For ultrasoft PPs you will need higher ecutrho due to the augmentation terms in the charge.

Try to read some literature about pseudopotential and ask to some experienced people 
working next door and, of course, experience yourself by trying.

HTH

GS


> Thank you.
> 
> -- 
> Sincerely yours
> 
> Jiaye Li
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum

-- 


o ------------------------------------------------ o
| Gabriele Sclauzero, PhD Student                  |
| c/o:   SISSA & CNR-INFM Democritos,              |
|        via Beirut 2-4, 34014 Trieste (Italy)     |
| email: sclauzer at sissa.it                         |
| phone: +39 040 3787 511                          |
| skype: gurlonotturno                             |
o ------------------------------------------------ o