[Pw_forum] A question about force constants calculation output

[Eyvaz Isaev]

Dear Zhen,

In order to understand correctly the content of these files  you should read a textbook on the lattice dynamics theory of semiconductors.   Textbook "Fundamentals of Semiconductors" by Peter Yu and Manuel  Cardona is a very good starting point.

--- On Wed, 12/30/09, Zhen Huang <huang87 at purdue.edu> wrote:

> ph.x calculation generated files alas.dynX where X goes from
> 0 to 8. In the file alas.dyn0 it gives the k points.. Seems
> in the other dynX files for example .dyn1 it gives force
> constants of q=(0.0, 0.0, 0.0)  and .dyn2 gives force
> constants of q=(-0.25   0.25  -0.25) 

This is correct. 4x4x4 set of q-points gives 8 q-points for which dynamical matrices have to be calculated. These q-points are written in dyn0 file. Files dyn1 - dyn8 are dynamical matrices. 

> What is the T matrix  and Matrix 1 and 2 labeled below. 

There is no T-matrix at all, "T" means .true., i.e. AlAs, in fact, is semi-conducting. 

Next 3x3 matrix is the macroscopic dielectric constants matrix.

"Matrix 1" and "Matrix 2" contain effective Born charges for Al (atom 1) and As (atom 2). 
 
For metals you find "F" (.false.) and no dielectric matrix and Born effective charges.

> Also, the questions are what is 4 4 4 representing the the force 
> constants information part  labeled below,  what does the 1 1 1 1 mean > on the second line and also what is the meaning of the index  
> 2   1   1  -6.26636620736E-03 (for example), I assume -6.266e-03 is
> the force constants but what is 2 1 1. 

You asked this question before. Please have a look at 

http://www.democritos.it/pipermail/pw_forum/2008-September/010099.html

The only correction is that R=(i-1)*a + (j-1)*b + (k-1)*c (thanks Stefano d.G).

> If the forth column is the force constants why it is different from 
> Ph.x  calcualtion's output.

Please do not mix dynamical matrix with IFC. The first one is in q-space, and the second one is in R-space.
 
> One more question, if I want to look at the force constants
> between a unit cell and its closest nearest neibhor unit
> cell do I look at the dynX output with k=(0, -1, 0), (-1, 0,
> 0) and such and second nearest unit cells will be in dynX of
> k=(0, -.5, 0), (-.5, 0, 0)?
 
This is unclear what do you mean as IFC is for atoms, but not for unit cells. Just keep in mind in the Linear Response theory all calculations are performed inside the unit cell. To calculate the IFC range see a formula for R given above.

Bests,
Eyvaz.
 
 -------------------------------------------------------------------
Prof. Eyvaz Isaev, 
Theoretical Physics Department, Moscow State Institute of Steel & Alloys, Russia, 
Department of Physics, Chemistry, and Biology (IFM), Linkoping University, Sweden 
Condensed Matter Theory Group, Uppsala University, Sweden 
Eyvaz.Isaev at fysik.uu.se, isaev at ifm.liu.se, eyvaz_isaev at yahoo.com