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

[Nicola Marzari]

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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Prof Nicola Marzari   Department of Materials Science and Engineering
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