# [Pw_forum] unit-printing error?

Stefano de Gironcoli degironc at sissa.it
Sat Mar 21 06:34:45 CET 2009

```there is no unit printing error.
remember that the relationship between direct and reciprocal vectors is
a(i)*b(j) = 2pi delta_ij
which is what you obtain when you use the printed quantities including
the declared units.
best regards,
stefano de Gironcoli

Quoting Karoly Nemeth <nemeth at anl.gov>:
>
> Hi,
>
> I became aware of some unit printing errors in PWSCF (v.4.0.4). At least
> it seems to be case.
> In one of my calculations I got:
>
>       crystal axes: (cart. coord. in units of a_0)
>                 a(1) = (  2.556193  0.000000  0.000000 )
>                 a(2) = (  0.000000  2.556193  0.000000 )
>                 a(3) = (  0.000000  0.000000 30.000000 )
>
>       reciprocal axes: (cart. coord. in units 2 pi/a_0)
>                 b(1) = (  0.391207  0.000000  0.000000 )
>                 b(2) = (  0.000000  0.391207  0.000000 )
>                 b(3) = (  0.000000  0.000000  0.033333 )
>
>
> Obviously, the reciprocal axes is printed in units of 1/a_0, and not in
> "2 pi/a_0". A few lines below in the output, where k-points are listed
> I see:
>
>       number of k points=  260  gaussian broad. (Ry)=  0.0300     ngauss =
> -1
>                         cart. coord. in units 2pi/a_0
>          k(    1) = (   0.0000000   0.0000000   0.0000000), wk =
> 0.0039063
>          k(    2) = (   0.0000000   0.0000000   0.0041667), wk =
> 0.0078125
>
> Comparing reciprocal lattice to the listed k-points, it turns out, that
> k-point are also listed in units of 1/a_0, and not in "2pi/a_0".
>
> This seems to be a unit-printing error, as otherwise everything is
> consistent. If I remember well, in older versions the unit used to be 1/a_0 .
> Please let me know if I am right with the above.
> Thanks:
>
> Karoly
>
>
> ***********************************************************************
> Karoly Nemeth, Ph.D.
> Argonne National Laboratory
> 9700 S. Cass Ave. Argonne, IL 60439.
> Advanced Photon Source
> Accelerator Physics and Operations Group
> Building 401, Room B2200
> Tel. 630-252-5813
> Fax: 630-252-4732
> ***********************************************************************
>
>
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