# In VASP, which option is better for monolayer relaxation: ISIF=3 or ISIF=4?

Since I have started working on monolayers I was using the ISIF=3 tag for relaxation, this was after recompiling VASP with the modified "constr_cell_relax.F" file. There was no problem in relaxing pure materials, but with the doping of 4x4x1 supercells I don't get convergence in relaxation and the calculations are taking too too much time. On the other hand my friend is using the tag ISIF=4 and he is getting always converged results.

Now I am a little bit confused with which tag should be used, since I think that because of vacuum along the c axis there should be no relaxation along it, so ISIF=4 tag shouldn't be used. I want to know your opinions guys especially the ones who worked on relaxing doped monolayers.

• The convergence problems may be related to ALGO and IBRION, rather than ISIF. – Jack Feb 17 at 4:31
• So which ones should be used for monolayers with vacuum along c axis? – Chi Kou Feb 17 at 7:58
• Take a look at this post: mattermodeling.stackexchange.com/questions/2022/… – Jack Feb 17 at 10:46
• By the way, the ISIF=3 tag with recompiled VASP [ modified constr_cell_relax.F] is suitable for monolayer relaxation. – Jack Feb 17 at 10:48
• Also this post: mattermodeling.stackexchange.com/questions/2032/… – Jack Feb 17 at 10:49