[ase-users] Minimum k-point density

[Peter Bjerre Jensen]

(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>


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listserv.fysik.dtu.dk/pipermail/ase-users/attachments/20150306/083fd0af/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: DTU_email_logo_01.gif
Type: image/gif
Size: 1055 bytes
Desc: not available
URL: <http://listserv.fysik.dtu.dk/pipermail/ase-users/attachments/20150306/083fd0af/attachment.gif>