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

Giovanni Cantele giovanni.cantele at spin.cnr.it
Wed May 18 10:14:44 CEST 2016

```> On 18 May 2016, at 09:54, Dae Kwang Jun <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?

idir=3 means that the planar average is performed within planes defined by a1 and a2 (first two Bravais lattice vectors). So, in the case of the tetragonal lattice, ibrav=6, you have
v1 = a(1,0,0),  v2 = a(0,1,0),  v3 = a(0,0,c/a)
so idir=3 just means planar average within planes parallel to xy.

> And does it follow the same logic in other lattices?

the logic is that explained above

> Lastly, does awin refer to the area of the plane? Or does it refer to the thickness of the plane?

Neither. awin is the size of the macroscopic average. Once the planar average has been performed, you are left with a function of a single variable, say f(z) (in the above example, for tetragonal
lattice and idir=3).  f might describe an electrostatic potential, charge density of any other three-dimensional field associated to your system. It turns out that usually f(z) does show microscopic oscillations, for
example in the case of the electrostatic potential you will see alternating maxima and minima in correspondence of lattice planes or vacuum space between them. You might want to get rid of such macroscopic
oscillations, that might be useful for example in work function calculations, were you want to define a “constant” reference level of the potential within your structure. In this case you fix awin to the distance
between lattice planes (similar to average a sin(z) over a period), that almost eliminates all the oscillations in the bulk part of the structure. Beware: if I well remember, awin is in atomic units (check if this is correct).
>
>
> Dae Kwang Jun

Giovanni

--

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: http://people.na.infn.it/~cantele

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