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

[ma...@gmail.com]

Hi Nick,

Thank you very much for your suggestions! I will run some tests.

Thanks&Regards,
Hongyang

在2021年4月9日星期五 UTC+10 上午11:47:59<n... at berkeley.edu> 写道：

>
> The lattice parameter obtained by VASP HSE06 with kpoints and a 
> hypothetical cp2k HSE06 calculation with kpoints should be very similar 
> because at that point (assuming converged basis sets) the only difference 
> is the pseudopotential and both vasp and cp2k have quality 
> pseudopotentials. So I would say they should be fairly commensurate. There 
> will be a small discrepancy beyond pseudopotentials because your supercell 
> for ADMM will be equivalent to coarser k-point mesh, but since the 
> pressure-volume energy is quite small, it shouldn't affect your results too 
> much.
>
> Using different methods shouldn't matter very much as long as they are 
> both converged. They are just they way to solve the SCF. They should 
> converge almost identical energies so long as the system has a band gap to 
> ensure OT is appropriate.
> On Thursday, April 8, 2021 at 6:26:21 PM UTC-7 ma... at gmail.com wrote:
>
>> 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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