Guidance on Force Convergence

[shaun_tay]

Hello,

I am currently running a geometric relaxation calculation for a ZnFeC6N7H4 supercell with 96 atoms. Despite setting a force convergence criteria of 0.005 eV/A, the maximum force remains above 0 (positive value). Here is what I have attempted:

• IBRION 1 and 2: It appears that IBRION 2 performed better with a lower converged total energy as compared to IBRION 1

• Increased EDIFF from 1e-5 to 1e-8: This adjustment did not change the forces

It appears that re-running the calculations again with the previously relaxed structure reduces the forces slightly (as expected). However, the forces still remains above 0 eV/A.

Could someone provide guidance on the following criteria for convergence? Thanks!

Attached are the input files and force changes.

[christopher_sheldon1]

Hi Shaun,

Thank you for your question. Positive forces is not an issue. When set to a negative value, EDIFFG converges the norm of the forces to within 0.005 eV/A. The negative does not mean that the forces themselves should be necessarily positive or negative, only their norm. If EDIFFG is positive, then the energy difference between ionic steps is converged. It looks like the magnitude (hence always positive) of your forces converges to below the threshold in your figure.

Does this answer your question?

Best wishes,

Chris