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

[ma...@gmail.com]

Hi Nick,

Thank you very much! This helps a lot!
Regarding the first question, I compared the lattice parameters abtained 
from VASP and cp2k both using PBE functional and the results are basically 
same (maybe ~0.0001 A difference which I think is negligible). So do you 
think using the lattice parameters getting from VASP HSE06 calculation with 
multiple k-points as the lattice parameters for cp2k HSE06 would work too?
And another question if I may ask is that for PBE calculations, I tested 
different methods (e.g., DIAGONALIZATION and OT with different MIXING and 
MINIMIZER) and found that DIAGONALIZATION with Pulay Mixing is the most 
efficient. But clearly this won't work for HSE06 for the large system. So 
based on your expertise, do you think I'd better use totally same methods 
(i.e. OT) for both PBE and HSE calculations rather than using different 
methods for PBE and HSE (i.e., DIAGONALIZATION for PBE and OT for HSE, 
respectively)?

Thanks&Regards,
Hongyang

在2021年4月9日星期五 UTC+10 上午10:59:28<n... at berkeley.edu> 写道：

>
> 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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