Increase of the SCF cycles along an AIMD run

[Dune André]

Dear all,

I am facing an issue while running AIMD simulations of a box of ~1000 atoms. The system is an amorphous MOF made of Fe linked by benzene tricarboxylate (named Fe-BTC).

I use PBE functional with DZVP basis set and GTH pseudopotential ; DIIS method and FULL_KINETIC preconditioner. I perform the calculation from a restart file. The first steps of the MD are quite fast with ~10 SCF steps per MD step. But along the AIMD run, the SCF cycles become longer and longer (~100 SCF steps per MD step).

I tried to use other OT method such as CG or other preconditioner such as FULL_SINGLE_INVERSE but in both cases the SCF cycles are taking too much time and sometimes fail to converge. Besides, FULL_ALL preconditioner is too computationally expensive in terms of memory.

Do you have some ideas of the origin of this increase of the SCF cycles? I attached the restart file to the message.

Thanks in advance for your help,

Kind regards,

Dune André

[Lucas Lodeiro]

As a first glance, your EPS_DEFAULT is 10⁻⁷ which is too big, I usually use 10⁻¹⁰ at least, or 10⁻¹² for more precise AIMD. Also a good relation between EPS_DEFAULT and EPS_SCF is to set the last one as the square root of the former.

All this affects the quality of the forces which dominates the MD... maybe if you check the geometry of the long SCF frames you could find some weird bond lengths or unphysical structures which can explain the behaviour.

Regards - Lucas

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[Dune André]

Hi Lucas,

Thanks for your answer. I have tested to increase the SCF convergence thresholds as you mentioned it: one calculation with (EPS_DEFAULT=10-10 et EPS_SCF=10-5) and one calculation with (EPS_DEFAULT=10-12 et EPS_SCF=10-6). In the former calculation, I had the same problem (increase of the SCF cycles along the trajectory) while in the latter SCF did not converge.

I tried improving SCF convergence by using an outer SCF loop but it was unsuccessful. The trajectory does not look unphysical, I don't see weird structures.

It seems that the increasing number of SCF cycles comes from the combination of DIIS method and FULL_KINETIC preconditioner (maybe due to a numerical effect?) but I am stuck to overcome this difficulty. Do you see other potential reasons of this behavior?

Thanks a lot.

Best regards,

Dune

[captain mus]

Hi Dune 

The origin of the increase of the SCF cycle in principle is because the calculation still trying to find the best wave functions, this is why if you look at your output (grep time out.out) the first step of SCF convergence always takes longer than the later. Since you start from the restart file, assume that you have the wfn file (i.e., file-RESTART.wfn) then you can use it to decrease the SCF cycle and speed up the calculation with the following
-------------------------------------------------------------------
&SCF
       ....
           SCF_GUESS  HISTORY_RESTART
      ....
&END SCF 
-------------------------------------------------------------------

Hope it help 

Best
Saleh