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[Razvan Caracas <caracas@pcpm.ucl.ac.be>]

Dear Martin,

rprim gives you the projection of the acell on the cartesian axes.
so considering only the interesting first line of rprim, we have
rprim 1.0 0.5 0.0 means a x-axes projection of 1 and y-axes projection 
of 0.5, which will give you a shear of 26.56 degrees
then
rprim 1.0 1.0 0.0 means a x-axes projection  =  y-axes projection, and 
so you get your 45 degrees shear
(I suppose your NiTi is cubic !)

Sincerely,
Razvan



M.G.A.Tijssens wrote:

> Dear abiniters,
> I have been calculating elastic constants for NiTi. One deformation
> mode of the unit cell that I use is a simple shear. To accomplish this
> I include in the input file the lines:
>
>
>
> rprim: 1.0    0.0    0.0
>       0.0    1.0    0.0
>       0.0    0.0    1.0
>
> rprim+ 0.0    0.02   0.0
>       0.0    0.0    0.0
>       0.0    0.0    0.0
>
> As I understood, these give the directions of the vectors spanning
> the unit cell and they are multiplied by the dimensions given in
> acell (which is three times 5.718 in my case, corresponding to the
> minimum energy lattice parameter found for Troullier-Martins
> potentials).
>
> The results that I anticipated is that the energy will increase, and
> reach a maximum when (the first line of) rprim equals 1.0 0.5 0.0
> since at that moment the unit cell has been sheared by 45 degrees.
>
> However, the maximum is reached when the first line reads 1.0 1.0 0.0.
> Can anybody tell me if this is a mistake in abinit (doubtfull...)
> or my misinterpretation of rprim. If so, how then should I interpret
> rprim (and I did read the help file for this variable!).
>
> Thanks for any suggestions,
> Martin Tijssens 

--
M. Razvan Caracas
Universite Catholique de Louvain,
Unite de Physico-Chimie et Physique des Materiaux
Bat. Boltzman, 1, pl. Croix du Sud B-1348 Louvain-la-Neuve, Belgique
tel: 0032-(0)10478191  fax: 0032-(0)10473452
e-mail: caracas@pcpm.ucl.ac.be      
http://www.mapr.ucl.ac.be/~caracas/index.html