The ABINIT Users Mailing List ( CLOSED )

["M.G.A.Tijssens" <M.G.A.Tijssens@LR.TUDelft.NL>]

Dear Razvan,

yes, that is exactly how I understood it, i.e. the rprim gives
the components of the vectors spanning the unit cell in terms
of the cartesian unit vectors. But if I have a cube with atoms
on the corners that is sheared by the 26.56 degrees, as you
correctly indicate, then I should reach my maximum energy at this
shear angle. Shearing further should decrease my energy again.
Abinit is NOT reproducing that behavior! So, once again I ask,
could this be a mistake of abinis, or am I missing something else?

Best regards,
Martin


Razvan Caracas wrote:
> 
> 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

-- 
Dr. M.G.A. Tijssens               T: +31 15 278 9552
Delft University of Technology    F: +31 15 261 1465
Dept. of Aerospace Engineering    E: M.G.A.Tijssens@lr.tudelft.nl
Postbus 5058
2600 GB  Delft
The Netherlands