density mesh cutoff

[Cyrille]

Dear atk experts

I have recently faced the following problem when working on MOF (metal-organic framework).
My system contains 3 metallic ions (Cobalt) which are strictly equivalent (when in "ideal" position: see image).  However during the scf loop the 3 atoms sometimes acquire different charges and magnetization which can in some cases have a rather drastic influence on the electronic structure.
I manage to "solve" this problem by increasing the density mesh cutoff (up to 150 Hartree).

Are there other possible options to solve this issue?

Cyrille

[Petr Khomyakov]

In general, one has to converge the density matrix with respect to the density mesh cutoff. It might be that you have just found that the density matrix converges for your particular system at that high cutoff value.

You may also try increasing reciprocal_energy_cutoff from its default value of 1250 Ha to something like 5000 Ha,  see https://docs.quantumwise.com/manuals/Types/NumericalAccuracyParameters/NumericalAccuracyParameters.html. This cannot be done in the VNL, i.e., you should modify the python script.

[Cyrille]

OK thanks for your reply. In terms of computational cost what is the "best" solution?
increase density mesh cutoff or reciprocal_energy_cutoff
Cyrille

[Petr Khomyakov]

In principle, the strategy is to converge with respect to both parameters. In practice, you may first increase the reciprocal_energy_cutoff value for a given density mesh cutoff to see if that eliminates the artifact. If so, you can still increase the density mesh cut off to see whether the quantities of interest are converged with respect to the mesh cut off.