[Pw_forum] Stress tensor from pw.x and cp.x

[Lawrence Lee]

Dear Nicola,

Thank you very much for giving me some ideas about the issue! Indeed, I
am using ultrasoft pseudopotentials. And therefore, I've set the nrb's
carefully using the formula, "nr1b=2*rcut*nr1/Lx", and make it a bit
larger than values calculated with this equation. However, you remind me
that I may not have a high enough ecutrho. My ecutwfc is 35 Ryd and 150
for ecutrho. This might not be enough...

I will try a higher value on ecutrho to see if there are any difference.
If there is still the problem, I will then have to try the QE-4 package.

Thank you very much!

-- 
S.H. Lee
M.Phil
Physics Division
The CUHK Graduate School




On Thu, 2008-05-22 at 02:36 -0400, Nicola Marzari wrote: 
> 
> Dear Lawrence,
> 
> 
> the only suggestions that comes to mind is this - the stress tensors
> converges to its "true" values more slowly with energy cutoff then
> other quantities. Usually the error at standard cutoffs is
> fairly "rigid", i.e. due to contributions that are not sensitive to
> the chemical environment, and so you can even correct for that
> empirically.
> 
> CP and PWscf have a different way of treating augmentation charges,
> so that's the only major difference that comes to mind - but it would
> apply to ultrasoft pseudo (that's why posting your input would have helped).
> 
> Bottom line - let's use QE-4.0 (i.e. download it now) and retest
> with increasing cutoff, and if you are using ultrasoft pseudopotentials, 
> throw in for good measure a high charge density cutoff (10-12 times the
> wavefunctions). In CP, test also for the box grid size (nr1b...), 
> increasing values. And then maybe post again, at least the results
> and an input file. Of course, if it is ultrasoft, and your nr1b is not 
> really that good, that could explain it right away.
> 
> 
> 				nicola
> 
> 
> 
> Lawrence Lee wrote:
> > Dear all,
> > 
> > Since my calculation is really simple, I am not going to attach my input
> > file here. I use the QE-3.1.1 code, and perform scf calculation using
> > pw.x on a 64-atom cell, which has a perfect fcc structure, using only
> > the gamma point. I obtain a stress tensor then.
> > 
> > Later on I use exactly the same structure and cell and cut-off's, but
> > using cp.x to perform a damped dynamics on the electrons, keeping the
> > atoms fixed. I also obtain a stress tensor.
> > 
> > However, the results between pw.x and cp.x is different, apart from the
> > difference in units. The cp.x results are generally about 5~10 GPa above
> > that of pw.x.
> > 
> > I find this quite confusing. Is the method of calculation of stress
> > tensor by these two programmes different?
> > 
> > Thank you very much for solving my doubt.


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