how to relax cell under specific pressure

[linglin]

dear sir,
i wonder if i can relax the cell under a specific pressure in VASP?
how to do it?

sincerely,
tom
5/20/2009

[forsdan]

http://cms.mpi.univie.ac.at/vasp/vasp/node118.html

[forsdan]

http://cms.mpi.univie.ac.at/vasp/vasp/node118.html

[linglin]

Thanks! but that tag seems only govern the puley stress, i wanna a specific external stress, how it would work? could you explain it to me in some detail?

[forsdan]

The PSTRESS tag adds the specified stress to the diagonal elements of the stress tensor. Pulay stresses on the other hand are connected to how accurate the stress tensor itself is calculated due to the finite basis set. There is a distinct difference, although the Pulay stresses typically tend to be isotropic in nature.

So first you should make sure the stress tensor is accurate enough by using PREC=Accurate and gradually increase the cutoff energy until you reach convergence for the stress tensor. This will make the Pulay stresses sufficiently small. THEN you specify your external pressure using the PSTRESS tag while keeping the high ENCUT
value and PREC = Accurate.

In theory, an alternative is also that you first estimate the Pulay stresses by the procedure given in the pdf-talks to get the PSTRESS correction for this. Se page 10 in

http://cms.mpi.univie.ac.at/vasp-worksh ... curacy.pdf

Then you can add the external pressure to the PSTRESS tag as well, so that PSTRESS = Correction for Pulay stresses + External pressure you want to simulate. This will allow for a lower cutoff energy, but it's less accurate.

Best regards,
/Dan




<span class='smallblacktext'>[ Edited Wed May 20 2009, 01:41PM ]</span>

[linglin]

i did as you told and it works quite well!
Thank you very much!

cheers
tom