Static calculation doesn't yield a close result to the previous relaxation run.

[mikehp]

Hi,
After the ionic relaxation, my structure comes to a very nice equilibrium state with the external pressure close to zero. Then I bring that output structure file (CONTCAR) to a static calculation (ISIF=2, IBRION=-1, NSW=0) , however after the static calculation, the external pressure shows to be very high (for instance, 30 kB), while it is supposed to be around zero as of previous step. I never had this problem before, so what might be the reason? A lot of rerunning doesn't help. My only doubt is that CONTCAR file, somehow in this case, doesn't contain the final version of that relaxed structure.
Are the values of TOTEN energy after relaxation step in OUTCAR file exact so that I can neglect the static calculation step?
Thank you

[forsdan]

If you carry out volume relaxations you will have to consider the possibility of Pulay stress:

http://cms.mpi.univie.ac.at/vasp/vasp/node209.html


I.e,, If your cutoff energy and precision are too small the corresponding stress tensor will be incorrect and thus the pressure will be misleading. When you restart your static calculation you adopt a new spherical basis set and the energy and stress tensor will change.

So you should carry out the volume relaxation with an increased cutoff (at least 30%) and increase the PREC-tag to the highest precision in order to get accurate volume relaxation.

Cheers
/Dan


<span class='smallblacktext'>[ Edited Fri Aug 22 2008, 07:21AM ]</span>