Dear Stefano, I have read the link you show. Thank you very much. I actually used high Ecut (35Ry). The results I sent in the previous email are for Ecut-35Ry. Let me check everything again to see if I made any mistake.
Best, Trinh > Dear Trinh Vo, > this has been explained a number of times in this forum. > The way stress is calculated assumes that the number on plane > waves does not change during the deformation. > This is NOT what you are doing when calculating energy vs alat > curves at fixed CUTOFF ... which explains the difference between the two > results. > Stress converges slowly with cutoff. To be reliable calculations > involving stress should use higher cutoff than usual. > > Have a look at the attached slides that try to clarify the issue. > These were presented in a Summer School in Santa Barbara in 2009 > whose material can be accessed at > http://media.quantum-espresso.org/santa_barbara_2009_07/index.php > > best > stefano > > vtmtrinh at caltech.edu wrote: >> Dear All, >> >> I am running a test calculation on a cell with 64 Si atoms. I first run >> a >> series of scf calculations of different cell sizes (the atoms position >> coordinates are also generated accordingly with cell dimensions by my >> code). I got the following results: >> >> a (A) Total E(Ry) >> 10.775 -508.1653034 >> 10.770 -508.1653391 >> 10.76932 -508.1653368 >> 10.765 -508.1653067 >> 10.76 -508.1652012 >> >> From the above results, it looks like to me I would have a min E close >> to/at a=10.77A (plotting E vs a give me a nice parabola curve). I then >> rerun the structure at min E (i.e. a=10.77) using vc-relax. I obtained >> the following: >> >> CELL_PARAMETERS (alat) >> 1.000000000 0.000000000 0.000000000 >> 0.000000000 1.000000000 0.000000000 >> 0.000000000 0.000000000 1.000000000 >> >> >> alat = 20.3289 au (or 10.75760138 A), E=-508.1650690 Ry >> >> Comparing to the results by scf calculations, I see the result from >> vc-relax and scf calculations are not consistent at all. I thought they >> should. However, the total stress computed for the scf calculation at >> min >> energy is still large (P = -3.36 kbar), while the stress calculated by >> the >> vc relax is small P = -0.04 kbar. I also noted that the cell size at >> which the system has a min stress (i.e. almost zero stress) given by scf >> calculations does not coincide with the cell size at which the the >> system >> has a minimum energy (min of the parabola). I thought that at the >> optimized cell size, both stress and energy have to be minimum. I don't >> understand why there is such a difference. Am I wrong in the way I was >> doing? >> >> Thank you very much, >> >> Trinh Vo >> JPL/Caltech >> >> >> >> >> >> _______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org >> http://www.democritos.it/mailman/listinfo/pw_forum >> > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum >
