Dear Stefano, Thank you very much for your help.
Trinh > Dear Trinh, > considering that the bulk modulus of silicon is about 1 Mbar (1000 Kbar) > a convergence error in the calculated pressure of about 3 Kbar implies > an error in the volume of about 0.3 % which mean 0.1 % for the lattice > parameter which is what you have and is usually considered a good > accuracy. Other factors like XC functional and fitting procedure give > larger variations > If you want a convergence accuracy better that 0.1 % you need to > converge the pressure below a fraction of Kbar. > > stefano > > PS: as for the difference between the stress calculated by vc-relax at > the end of the relaxation and an scf calculation done at the converged > structure this is due to the fact that during vc-relax the calculation > is performed at fixed number of plane waves so that the two calculations > are not done with the same basis. > > > > > vtmtrinh at caltech.edu wrote: >> 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 >>> >>> >> >> >> _______________________________________________ >> 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 >
