[CP2K:8369] BAND_STRUCTURE k-point unit conversion bug leads to bogus band structures (CP2K 4.1)

[Ari Paavo Seitsonen]

Hello,

  Talking about the k points and their units - would it be possible to have
an option for 'MACDONALD' scheme of k points where the "kp_shift" would
also be given in units of the reciprocal lattice vectors - they are natural
due to the algorithm for their construction and easier - or even an
option/short-cut that would enable giving Gamma-point centred k point sets,
as these are needed in hexagonal cells. Now the shift depends on and thus
changes with the chosen number of grid points.

  A side note - in the documentation there appears to be a small error,
namely the shift is not mentioned together with the 'MACDONALD' scheme, yet
the code crashes if one only provides the three integers mentioned in the
document (this is correct for the scheme 'MONKHORST-PACK'). And the
symmetries apparently always fail with the 'MACDONALD' scheme - as said,
this is basically the compulsory scheme in hexagonal cells. And if I may
note, I tried to calculate a hexagonal cell together with the 'MACDONALD'
method, with a shift adjusted so that Gamma point is included in the set,
but I obtain a strange reduction of symmetry in the forces (the three-fold
symmetry that is present due to the lattice is not obeyed laterally) - any
idea? I know that this is a new feature, maybe I should just be patient. :)

    Greetings from Paris,

       apsi

2016-11-09 10:14 GMT+01:00 <hut... at chem.uzh.ch>:

> Hi
>
> I updated the code for the k-point input. I hope I did catch
> all the error. I only made some minimal tests, so I am still
> not sure that it is correct.
>
> regards
>
> Juerg Hutter
> --------------------------------------------------------------
> Juerg Hutter                         Phone : ++41 44 635 4491
> Institut für Chemie C                FAX   : ++41 44 635 6838
> Universität Zürich                   E-mail: hut... at chem.uzh.ch
> Winterthurerstrasse 190
> CH-8057 Zürich, Switzerland
> ---------------------------------------------------------------
>
> -----cp... at googlegroups.com wrote: -----To: cp2k <cp... at googlegroups.com>
> From: Eric Hermes
> Sent by: cp... at googlegroups.com
> Date: 11/01/2016 09:35PM
> Subject: [CP2K:8332] BAND_STRUCTURE k-point unit conversion bug leads to
> bogus band structures (CP2K 4.1)
>
> Hello,
> I am working with ASE to develop routines for the various DFT calculators
> we support for simple, automatic generation of band structures. I have run
> into a bug in CP2K's SPECIAL_POINT specification, wherein the specified
> k-points appear to undergo a redundant B_VECTOR -> CART_BOHR unit
> conversion. What I mean by that is if I pass the k-points as multiples of
> the reciprocal unit vectors (i.e. UNIT B_VECTOR, SPECIAL_POINT 0.5 0.25
> 0.75 for the W point of the diamond lattice), this is multiplied by the
> reciprocal unit cell vector *twice*. If I pass the points in units of
> 2pi/Bohr (i.e. UNIT CART_BOHR, SPECIAL_POINT 0.049 0.098 0.000 for a
> primitive Si diamond lattice with lattice constant a=5.4 angstrom), the
> k-points are multiplied by the reciprocal unit cell vector *anyway*, giving
> the same results as before. I can "trick" the system into providing the
> correct result by saying that I am passing the k-points in 2pi/Bohr, but
> actually passing the k-points in multiples of the reciprocal unit cell
> vectors (i.e. UNIT CART_BOHR, SPECIAL_POINT 0.5 0.25 0.75). This results in
> both the output file and the band structure file printing the exact
> k-points that I specified (i.e. the k-points in multiples of the reciprocal
> unit vectors), and the resulting band structure matches that of other DFT
> codes.
> Here is an example of the input file I am using that expresses the bug:
> https://gist.github.com/ehermes/7ef3d7249b99eeafb34bb14e6c51fd48This
> input file was generated by (a modified version of) ASE, and it produces
> the incorrect band structure. If line 31 is changed to read "UNITS
> CART_BOHR", the resulting band structure is correct and matches the band
> structure produced by other codes (GPAW, VASP, etc).
> I have taken a look at the qs_band_structure.F source file, but the origin
> of the bug is not obvious to me. I suspect it might be related to line 197,
> which multiplies each kpoint by the transpose of the unit cell vector, but
> I am not certain.
>
>
>
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-- 
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  Ari Paavo Seitsonen / Ari.P.S... at iki.fi / http://www.iki.fi/~apsi/
    Ecole Normale Supérieure (ENS), Département de Chimie, Paris
    Mobile (F) : +33 789 37 24 25    (CH) : +41 79 71 90 935
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