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[Xavier Gonze <gonze@pcpm.ucl.ac.be>]

Dear scabuk,

Amm2 is face-centered orthorhombic, so the corresponding primitive  
vectors are not orthogonal.
The a b and c lengths that come from experimental data are those of the
conventional cell, and not the primitive one.
Typically, for the Cmmm Bravais lattice whose conventional cell sizes  
are (a,b,c), the primitive
vectors might be
(a/2  b/2 0)
(a/2 -b/2 0)
(0     0     c)

Good luck,
Xavier




On 30 Apr 2007, at 11:55, scabuk@cu.edu.tr wrote:

> Dear abinit users,
>
> Nowdays, I would like to calculated some structure parameters the  
> orthorhombic phase Amm2 (38) of KNbO3. The lattice constans (a,b,c)  
> and atomic position are the experimental data (ref. A.W. Hrwat, J.  
> Phy. C:solid state Phys. vol.6,pp.2559,1973).  I try alot of  
> probobility, e.g. I change the order of lattice constans from  
> (a,b,c) to (c,a,b) and also xred, yred,zred and changed atomic  
> position, etc. But I couldn't get the space group Amm2. I can use  
> LDA pseudotantials. Output file was given me the space group Pmm2  
> (25).  How can I get the space group Amm2 ? Please help!
>
> Thanks in advance.
>
> Best Regards.
> e-mail:scabuk@cu.edu.tr
>
> my input file:
> #Definition of the k-point grids
> kptopt 1
> nshiftk 1
> shiftk  0.5 0.5 0.5
>
> ngkpt  4 4 4
> getwfk -1
>
> #Definition of the unit cell
>
> acell 7.5041 10.7601 10.8092
> #rprim  1.0  1.0 0.0
>        -1.0 1.0 0.0
>        0.0  0.0 1.0
>
>
> angdeg 90 90 90
>
> #Definition of the atom types
> ntypat 3
> znucl 19 41 8
>
> #Definition of the atoms
>  natom 10
>  typat 2*1 2*2 6*3
>  xred
>    0.0    0.0  0.0138
>    0.0    0.5  0.5138
>    0.5    0.0  0.5
>    0.5    0.5  0.0
>    0.0    0.0  0.4636
>    0.0    0.5  0.0364
>    0.5   0.7524 0.7158
>    0.5   0.2476 0.7158
>    0.5   0.2524 0.2158
>    0.5   0.7476 0.2158
>
>
> #Definition of the planewave basis set
> ecut  35.0
>
> #Definition of the SCF procedure
> nstep 1000
> toldfe 1.0d-6
>
> output file:
> Symmetries : space group Pm m 2 (# 25); Bravais oP (primitive ortho.)
> ====================================================================== 
> ==========
>  Values of the parameters that define the memory need of the  
> present run
>    intxc =         0  ionmov =         0    iscf =         5      
> ixc =         7
>   lmnmax =         4   lnmax =         4   mband =        26   
> mffmem =         1
> P  mgfft =        60   mkmem =         8 mpssoang=         4      
> mpw =      8647
>   mqgrid =      1201   natom =        10    nfft =    144000     
> nkpt =         8
>   nloalg =         4  nspden =         1 nspinor =         1   
> nsppol =         1
>     nsym =         4  n1xccc =      2501  ntypat =         3   
> occopt =         1
> ====================================================================== 
> ==========