[CP2K-user] How to check the convergence of ADMM

[Nicholas Winner]

Hi Hongyang,

(1) Currently, you are right, HFX needs to use only gamma point. Therefore, 
instead of k-points you have to rely on supercell approach in order to 
converge the properties. One can get good properties using supercells of 
sufficient size. For example, a Si primitive cell with 2 atoms and 8x8x8 
k-points would be roughly equivalent to a 1024 atom cell at gamma point 
only. Alternatively, you can run smalleer cell and still get good results. 
I would guess that ~250 atoms will give good lattice constants for Si. If 
you *must* perform cell optimization using hybrid, then this is what I 
would suggest; however, even using ADMM that would be a costly calculation. 
Bulk properties like lattice constants are often correctly predicted by 
GGA, and so you might consider simply optimizing the lattice constants at 
the GGA level and then passing the calculation to HSE06 after. This will 
depend on your system, but give it some thought. 

(2) This is a little opaque yes. In general, I believe that you don't have 
to worry about the convergence of the ADMM basis set for 3 reasons: (1) the 
ADMM has been demonstrated to perform well in many papers in the 
literature, and so the use of ADMM will not be questioned by many these 
days. (2) checking ADMM against the primary basis is absurdly expensive, to 
the point where it is often impossible to check. You can check the 
convergence of just the auxiliary bases by comparing the energies different 
auxiliary basis sets to one another in a series of static calculations if 
you want to be thorough. (3) You have already committed to sacrificing a 
tiny bit of accuracy by using the ADMM instead of the primary basis. It's 
the compromise you make in order to run large calculations, and so you 
shouldn't expect it to perfectly reproduce the primary basis, but it will 
do a decent job.

(3) The band gap can be read off by setting the SCF solver to OT and then 
turning on FORCE_EVAL%DFT%PRINT%MO_CUBES. A minimal working example of what 
I mean:

&FORCE_EVAL
    &DFT
        &SCF
            &OT
            &END
        &END
        &PRINT
            &MO_CUBES
                NHOMO -1
                NLUMO -1 
            &END
    &END
&END


Hope this helps.
-Nick
On Thursday, April 8, 2021 at 5:07:37 PM UTC-7 ma... at gmail.com wrote:

> Hi,
>
> I am a rookie in using cp2k. In my project I need to use HSE06 for a large 
> system (1000 atoms) calculation. I have a few questions about using HSE06. 
> Could somebody provide me some suggestions?
> (1) In pure DFT calculations (such PBE), I can use MONKHORST-PACK kpoints 
> scheme (e.g. 8 8 8) and run CELL_OPT calculations to get the optimized 
> lattice parameters and then compare these parameters with literatures to 
> confirm that the BASIS_SET and other settings used in the cp2k calculation 
> are good enough to reproduce this sytem. However, in HFX calculations, it 
> looks we can only use GAMMA kpoints scheme. The CELL_OPT with single kpoint 
> can not get the correct lattice parameters. Then how can we confirm the 
> BASIS_SET and setting in the cp2k calculations are accurate enough?
> (2) HFX calculations are highly expensive so using ADMM approach is 
> necessary. In the tutorial, it says "Always check the convergence of the 
> primary and ADMM basis sets". I'm wondering what does this mean and how 
> exactly should we do the check the convergence of the ADMM basis set? Or in 
> another word, how do we usually choose which ADMM we use for our 
> calculations?
> (3) My material is Si, a semiconductor. I'm wondering is it possible that 
> I can read the band-gap value directly from the output file after the SCF 
> calcualtion?
> I really appreciate it if somebody could provide some help.
>
> Thanks&Regards,
> Hongyang
>
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