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. _______________________________________ ?x?????? - Yahoo! Messenger ???????]?????W?????????????????????????o???????????W?r???????????????????f???????????? http://messenger.yahoo.com.hk