# [Pw_forum] INPUT file for average.x

Gabriele Sclauzero sclauzer at sissa.it
Mon Apr 26 08:41:20 CEST 2010

```
wulilixiangguo at sina.com wrote:
>  I read below and have a question about macroscopic averages. If my
> system have a surface between two materials which have different lattice
> constant.

Do you mean an interface? (I would say a "surface" only if one of the two "materials" is
vacuum, but maybe I'm pedantic...)

How to set the awin?

Not sure of this, but I think that it is not a good idea to use macroscopic averages when
there is a change in the periodicity. For instance, in a recent paper by Singh-Miller and
Marzari (PRB, 2010) it has been reported that the technique is not well suited for slab
geometries when you account for the relaxation of the distances between layers.

How to macroscopic it twice?

Maybe I don't understand this point.

You could run the program twice, with two different window sizes, in order to match the
two different lattice spacings, but then I don't know if there is a way to give a
meaningful interpretation to the data at the interface.

GS

>
> Thank you!
>
> 1
> filename
> 1.0
> 3000
> 3
> 2.59
>
> The explanation is in the header of the file PP/average.f90:
>
>   !      nfile        ! the number of 3D-FFT files
>   ! for each file:
>   !      filename     ! the name of the 3D-FFT file
>   !      weight       ! the weight of the quantity in this file
>   !      .
>   !      .
>   ! end
>   !      npt          ! the number of points of the thick mesh
>   !      idir         ! 1,2 or 3. It is the fixed index which defines
>   !                   ! the planes of the planar average
>   !      awin         ! the size of the window for macroscopic averages.
>   !
>
> With the example above you'll get a 3-column output file for the
> quantity to average
> V(u):
> - column 1: coordinate u, defined over a mesh of npt=3000 points along
> the direction
> idir=3
> - column 2: averaged quantity V
> - column 3: macroscopically averaged quantity V', over a windows with size
> awin=2.50 au
>
> If you want to average a weighted sum of different quantities V1,...,VN
> defined over the same FFT mesh, just set nfile to N and specify N file names
> with the relative weight.
>
> Regards,
> Giovanni
>
>
>
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>
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| Gabriele Sclauzero, PhD Student                  |
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