# [Pw_forum] Clarification on idir and awin in average.f90

Giovanni Cantele giovanni.cantele at spin.cnr.it
Thu May 19 09:40:10 CEST 2016

```Al has a cubic lattice, so 100, 010 and 001 surfaces are all equivalent (so it is not a typo). On the other hand, idir DOES NOT refer to the surface orientation but to how it is oriented within the supercell.
Whatever are the surface orientation and the material you a re planning to study, you can define the surface supercell so that the vectors defining the surface unit cell are a1 and a2, whereas a3 represents the
direction orthogonal to the surface. In this case you can use idir=3 for studying 100,110 or 111 orientations. So, do not mistake the surface orientation you want to study with the supercell lattice vector that identifies,
in the pw.x calculation, that direction.

Giovanni

> On 19 May 2016, at 07:36, Dae Kwang Jun <jdaekwang at gmail.com> wrote:
>
> Dear Giovanni Cantele,
>
> Thank you once again. Also, there seems to be a typo in the example for workfunctions in PP/examples. The example states that the workfunction calculation is for Al100. However, its idir is set to 3. I think it should be Al001 instead. Lastly, I will check the units and put them here later.
>
> Sincerely,
>
> Dae Kwang Jun
>
> On Wed, May 18, 2016 at 4:54 PM, Dae Kwang Jun <jdaekwang at gmail.com <mailto:jdaekwang at gmail.com>> wrote:
> Dear all,
>
> I would like to make sure whether I am using these parameters correctly. I know that idir is the fixed index which defines the planes of the planar average. I also know that awin is the size of the window for macroscopic averages.
>
> However, I do not quite understand what is meant by these two statements. In the tetragonal lattice, it appears that idir=3 takes the average of planes that are perpendicular to the vector that has length = a. And that idir = 1 takes the average of planes that are perpendicular to the vector that has length = c. Is this correct? And does it follow the same logic in other lattices?  Lastly, does awin refer to the area of the plane? Or does it refer to the thickness of the plane?
>
> Thank you in advance,
>
> Dae Kwang Jun
>
>
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