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["Anglade Pierre-Matthieu" <anglade@gmail.com>]

Dear Waigen Zhang,

Chao Cao is right:
- ecut depends "only" on your pseudopotential. It never needs changes.
Unless you want a better precision (good) or a faster calculation
(evil);
- while k-point grid "spacing" depend on the cell size: as long as
your band structure do not change drastically, you can rely on the
variable "kptrlen" to monitor your k-point convergence. As long as
"kptrlen" is constant you keep the same precision for integration. If
it decrease, your precision decreases accordingly. Also, If, for
instance,  under pressure your system undergoes an insulator to metal
transition, your k-point grid will need to be redefined because a much
larger "kptrlen" will be required.

All this to say that Chao Cao is right. But I wanted to add the
following comment:
When one do a simple convergence study by varying ecut or kptrlen for
a single system, there is no guarantee that no error cancellation
happens. For instance, last summer some of my students with same
pseudopotentials found two different values for ecut to reach the same
"absolute" level of convergence. In fact, some of them where using the
FCC primitive cell and some other the cubic FCC cell. Also, the cell
parameters were slightly different. This adventure shows that if you
want absolute certainty about "absolute" level of convergence you'd
better test on more than a single system :-).

Best regards

PMA

On 10/4/07, Chao Cao <cao@qtp.ufl.edu> wrote:
> ecut defines your real space grid size, and it's irrelevant if you
> change the volume; however, the # of k-points might need change if you
> change the volume, but unless you change it a lot... I really don't see
> the necessity.
>
> Chao Cao
>
> waigen zhang wrote:
> > Dear all,
> > I am not sure if this problem asked  by somebody else or ...
> > As far as I know, ecut and kpoint convergence test is needed before
> > any physical calculation. How about if we keep changing the pressure
> > (volume)? Say if pressure=0 GPa. ecut=20,ngkpt=4X4X4 is enough. How
> > about when pressure=100 GPa (say volume is 50% of zere pressure
> > volume)? Should we
> > double check the ecut and ngkpt before any calculation?
> > If we need to check the convergence when volume changes(beyond the
> > ecutsm effect),is there any good way to do that?
> > Regards,
> > WG.ZHANG
>
>
> --
> Chao Cao
>
> Quantum Theory Project and Department of Physics
> University of Florida, Gainesville, FL 32611
> U.S.A.
>
>


-- 
Pierre-Matthieu Anglade