UT theoretical chemistry code forum

Dear professor,
When i did a calculation, I found it was impossible to attain Ediffg=-0.0001. Many methods had been used to improve the performance, such IBRION=1/2 and improving EDIFF (See att[attachment=0]OUTCAR.rar[/attachment]achment). However, it seemed to exist the limit of converge. Can you give me some advice to improve convergence precision and efficiency?
Thanks for your help and advice.
Yidan Wei

Here are input attachments.

My first question is why you want to converge a structure to such precision.

Then, you are using CG? You should be able to get more rapid convergence with LBFGS (IBRION=1) in the harmonic region.

But again, your energy is changing by 0.1 meV! Your faith in DFT should be 1000x less, with a typical uncertainty of 0.1 eV.

Dear professor,
I firstly used IBRION=1, however, it did not work, whose energy changed by 0.1 meV as well (See attachment). Therefore, i turned to IBRION=2 to seek a lower state.
Then, how can i improve performance？
Thanks for suggestion.
Yidan Wei

I want to calculate phonons 0.0.

The force and energy are both dropping systematically. Keep running. You only need to reach 0.001 eV/Ang or even 0.005 eV/Ang to calculate phonons.

Dear professor,
Thank you very very much. But Is the criteria enough to calculate phonon? I find Ediffg is always very, very low as the online instruction shows. And, I have other two questions, can you give me some advice?
Firstly, when i do geometrical optimization, should the ADDGRID = .TRUE. and LREAL = .FALSE. be added, as a 120 atom supercell ?
Secondly, What is the source of the imaginary frequency？Does the atom force matter？ Whether less force of atom can reduce imaginary frequency.
Thanks for your advice again.
Yidan Wei