[ase-users] [gpaw-users] Minimum k-point density

[Peter Bjerre Jensen]

Hi again

If no one disagrees about generating a minimum k-point density, could 
someone then change the code and submit it? (I do not have access)

Best regards
Peter

Peter Bjerre Jensen
Postdoc
DTU Energy

Technical University of Denmark

	
Department of Energy Conversion and Storage
Fysikvej, Building 309
2800 Kgs. Lyngby
pbjen at dtu.dk <mailto:pbjen at dtu.dk>
www.ecs.dtu.dk <http://www.ecs.dtu.dk>




Den 06-03-2015 kl. 16:48 skrev Morten Niklas Gjerding:
> Hi Peter.
>
> I don't have much to say about your first point other than you 
> argument sounds reasonable. On your other point:
>
> A monkhorst-pack grid not centered on the gamma point is not 
> guaranteed to fulfill the symmetries of non-orthorhombic unitcells. 
> This could be one reason on why to avoid even integers or to shift the 
> even monkhorst pack grid by using the {"gamma": True} keyword.
>
> I found something here (slide ~30) about the symmetry breaking: 
> https://www.vasp.at/vasp-workshop/slides/k-points.pdf 
> <https://www.vasp.at/vasp-workshop/slides/k-points.pdf>
>
> KR. Morten
> ------------------------------------------------------------------------
> *From:* gpaw-users-bounces at listserv.fysik.dtu.dk 
> [gpaw-users-bounces at listserv.fysik.dtu.dk] on behalf of Peter Bjerre 
> Jensen [pbjen at dtu.dk]
> *Sent:* Friday, March 06, 2015 4:38 PM
> *To:* gpaw-users at listserv.fysik.dtu.dk; ase-users at listserv.fysik.dtu.dk
> *Subject:* [gpaw-users] Minimum k-point density
>
> (crossposting to gpaw-users and ase-users, as both codes are involved)
>
> Hi
>
> According to the gpaw manual 
> (https://wiki.fysik.dtu.dk/gpaw/documentation/manual.html#manual-kpts) 
> one can specify a k-point density, instead of number of kpts per 
> direction:
>
>     You can also specify the *k*-point density in units of points per Å−1:
>
>     kpts={'density':  2.5}   # Monkhorst-Pack with a density of 2.5 points/Ang^-1
>     kpts={'density':  2.5,  'even':  True}   # round off to neares even number
>     kpts={'density':  2.5,  'gamma':  True}   # include gamma-point
>
>     The *k*-point density is calculated as:
>
>     Na2π,
>
>
> Normally I would expect that one was interested in specifying a 
> minimum k-point density, but that is not how the function works. (the 
> calls go all the way to ase.calculators.calculator.py)
>
>     def kptdensity2monkhorstpack(atoms, kptdensity=3.5, even=True):
>         """Convert k-point density to Monkhorst-Pack grid size.
>
>         atoms: Atoms object
>             Contains unit cell and information about boundary conditions.
>         kptdensity: float
>             K-point density.  Default value is 3.5 point per Ang^-1.
>         even: bool
>             Round to even numbers.
>         """
>
>         recipcell = atoms.get_reciprocal_cell()
>         kpts = []
>         for i in range(3):
>             if atoms.pbc[i]:
>                 k = 2 * pi * sqrt((recipcell[i]**2).sum()) * kptdensity
>                 if even:
>                     kpts.append(max(1, 2 * int(round(k / 2))))
>                 else:
>                     kpts.append(max(1, int(round(k))))
>             else:
>                 kpts.append(1)
>         return np.array(kpts)
>
> If one for example specify a cell = [6,6,6] and a kptdensity = 
> 25/(2pi), kpts is set to [4,4,4], which give a lower kpts-density (24 
> kpts per Å^-1).
>
> Wouldn't it make more sence to use int(ceil(***)), to ensure a minimum 
> density? (if so, the max(1,*) can also be skipped)
>
> Furthermore there is the option to only allow even integers - are 
> there any other reasons that one wants this, than to optimize the 
> parallelization, and it is recommend "always" to use even=True?
>
> Best regards
> Peter
> Peter Bjerre Jensen
> Postdoc
> DTU Energy
>
> Technical University of Denmark
>
> 	
> Department of Energy Conversion and Storage
> Fysikvej, Building 309
> 2800 Kgs. Lyngby
> pbjen at dtu.dk <mailto:pbjen at dtu.dk>
> www.ecs.dtu.dk <http://www.ecs.dtu.dk>
>
>


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