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

Hi again,

Do you relax your structure?  If your atoms are fixed by symmetry, then you
should adapt the acell (1) such as to keep the elongation 0.

Razvan

M.G.A.Tijssens wrote:
> 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
> >   

-- 
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