[Wien] Deformation of Diamond

[Stefaan Cottenier]

There is nothing wrong. You get what you ask for: if symmetry is
lowered, then the irreducible part of the first brillouin zone becomes
larger, and hence it contains more k-points.

You should ponder whether you really need such a low symmetry.

Even if you displace an atom over an infinitesimal distance only, you do
break symmetry (and 0.01 A is much larger than an infinitesimal
displacement).

What you really have to do is: take either the primitive cell, or the
easiest cell in which you can visualize the displacement you want.
Displace the atom you want. Execute sgroup (but without any labels for
the atoms, i.e. let sgroup determine the maximal amount of symmetry it
can find). Accept the result in case.struct_sgroup, and take this as new
case.struct. Do all calculations that are related to that particular
displacement in this cell, including the zero-displacement.

Stefaan


Hui Wang wrote:
> Dear wien2k master:
>     About questions last time, I adopted the suggestions of Kurt 
> Lejaeghere, here are the new problems:
>     (1)I changed the 8 atoms to C 1 , C 2,..........C 8, after the 
> sgroup, it found:
> =======================
> Bravais lattice: Triclinic
>      a             b            c
>  6.70248100    6.70248100   6.70248100
>      alpha         beta         gamma
>  90.00000000    90.00000000   90.00000000
> =========================
>      and also there was nothing wrong with symetry, I used all the 
> default setting parameters. Then to kgen:
> ===========================
> inputfiles prepared (07:16:03) 
>  >   inputfiles for lapw1c/2c prepared, no inversion present     
> (07:16:03) 
>  >   kgen        (07:16:03)            1  symmetry operations without 
> inversion
>  inversion added (non-spinpolarized non-so calculation)
>   NUMBER OF K-POINTS IN WHOLE CELL: (0 allows to specify 3 divisions of G)
> *10000*
>  length of reciprocal lattic vectors:   0.937   0.937   0.937  21.544  
> 21.544  21.544
>   Shift of k-mesh allowed. Do you want to shift: (0=no, 1=shift)
> *0*
> *then I get 4631 kpoints in IBZ, too much compared to cubic symetry 
> which only have 300 kpoints.*
> *then dstart -c I get 4698 FOURIER COEFFICIENTS CALCULATED UP TO GMIN, 
> also too much compared to cubic symetry which only have 80 FOURIER 
> COEFFICIENTS CALCULATED UP TO GMIN.*
> 
> *Here, my question are: Is there something wrong with the parameter ? I 
> know symetry is very low, but are 4631 kpoints in IBZ and 4698 FOURIER 
> COEFFICIENTS reasonable ? (I think this is too huge to calculate)*
> 
> *   *
>  
> *   (2)About the elastic constant, I konw when deform the structure will 
> change the spacegroup, but it should depend on the size you deform. 
> Here I just deformed 0.01 Ang in x axes, I don't think it will need a 
> new spacegroup.*
> *I also find *S. Jalali. already answered this problem long time ago:
> *=================================*
> The elastic constants can be derived from  second derivatives of the 
> total energy with respect to appropriate  deformations. The appropriate 
> deformations are generalized coordinates which  can be lattice 
> parameters or angles of the proper unit cells.
>   For cubic structure, where the matrix representation of elastic 
> tensors has  only 3 independent components, i.e.  C11, C12, C44=C66, you 
> can use the  elast package of wien2k, 
> http://www.wien2k.at/reg_user/textbooks/elast-UG.ps,  to calculate these 
> constants.
>   For other structure one must first find the proper deformations and 
> plot E  versus them and then try to evaluate the second derivatives of 
> the curves. 
>   Regarding your second question, I can say that, density of states 
> influences  not only the elastic constants, but also any other physical 
> quantities. This is  the density of state that differs from one case to 
> the others, and this is why  one case has different properties from the 
> others. Density of states in the energy-space  plays an important role 
> in materials, which can be compared to the role of  charge densities in 
> the real (coordinate)-space.
>  
>   Your,
>   S. Jalali.
> =====================================
> *   Here, my questions are: the hyperlink is forbidden although I signed 
> in with our account and* *password, I don't why ?  *
> *   For cubic system, how to deform it by only 0.01 Angstron in x axes 
> without changing the spacegroup?*
> ** 
> *   Any help will be appreicated:)*
> ** 
> *Hui*
> 
> 
> 
> 
> 
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-- 
Stefaan Cottenier
Center for Molecular Modeling (CMM)
Ghent University
Technologiepark 903 (2nd floor)
BE-9052 Zwijnaarde
Belgium

http://molmod.Ugent.be
email: Stefaan . Cottenier /at/ UGent . be