[QE-users] Optimal QE setup for molecular crystals (pseudopotentials, k-space campling, performance) ..

[Nicola Marzari]

Dear all,

the comparisons that Andrii mentions are all done here below, and very 
comprehensively - so it's just a matter of studying those and 
understanding them (it's not rocket science, and most of you are rocket 
scientists): https://www.materialscloud.org/discover/sssp/

I'll keep repeating this above forever, inspired by Cato the Censor.

Also, for lanthanides (a different post), you should be extremely 
careful - the ones I would trust are the Topsakal Wentzcovitch ones.

Nicola

On 17/11/2020 12:52, Andrii Shyichuk via users wrote:
> Dear Mihal,
>
> A few suggestions:
> 1). Compare a few types of PPs: PSLibrary (QE defaults), GBRV 
> (https://www.physics.rutgers.edu/gbrv/), ATOMPAW 
> (http://users.wfu.edu/natalie/papers/pwpaw/PAWDatasets.html).
>
> 2). The cutoff depends on the PPs, I'd not go below the value 
> recommended for the PPs used.
>
> 3). Are there any compounds chemically similar to yours, with good 
> known structures?
> If not - cell dimension is at least some quality indicator, good 
> dimensions mean good bond lengths, approximately.
> Smooth EOS (energy-volume curve) is an indicator that your setup is 
> self-consistent (basis is good, etc).
>
> 4). Gamma-only is not good for geometries. I'd use 3x3x3 grid (if your 
> systems is approximately a cube), or 2x2x2 (yes, K_POINTS automatic  2 
> 2 2 0 0 0 ) if 3x3x3 is too much.
>
> Best regards.
> Andrii Shyichuk, University of Wrocław
>
>
> W dniu 2020-11-16 22:46, Husak Michal napisał:
>> Hi all ...
>>
>>
>> Before I continue I would like to mentioned I had done the homework on
>> this topic (checked mail list, studied about 20 related articles, done
>> a few hundred test calculations by QE and CASTEP) ...
>>
>> The target of my work is to verify (enhance quality) of X-ray based
>> molecular crystal structure solution
>>
>> by DFT methods ...
>>
>>
>> Target molecules:  primary C,H,N,O composed middle size molecular
>> crystals (100-500 atoms per unit cell).
>>
>> Insulators. Non magnetic. Other atoms potentially: F,Cl,S,B,Na,
>>
>>
>> Goal: Confirm the geometry from X-ray is correct (by results RMS
>> comparison with original) ..
>>
>>
>> Questions:
>>
>> 1) What pseudopotential is suitable ? pbe-n-rrkjus_psl.1.0.0.UPF Like 
>> one ?
>>
>>
>> 2) Some sources suggest 38 Ry cutf off (study done in VASP),
>> H-ultrasoft potential suggest 46 Ry cutoff,
>>
>> QE related study (M. Lund) suggest 55 Ry cut off ...
>>
>> I need to screen a lot of structures = be fast = low cut off . Opinion ?
>>
>>
>> 3) How can i determine I had chosen correct parameters ?
>>
>> Comparison of RMS with X-ray data is not reliable - X-ray shows
>> position of max. electron density,
>>
>> DFT shows positions of cores (especially for H-atoms)  ...
>>
>> Any idea how to benchmark, before doing some large screening ?
>>
>> Does exist some database of "100% reliable determined" reference 
>> structures ?
>>
>>
>> 4) Gamma K-point only usage will be nice.
>>
>> M. Lund work suggest 2x2x2 k-point to be OK  (is it identical to
>> K_POINTS automatic  2 2 2 0 0 0 ) ?
>>
>> What do you thing ?
>>
>>
>> Thanks for any ideas
>>
>>
>> Michal Husak
>>
>>
>> UCT Prague
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