[Wien] Do I should accept the struct generated by sgroup?

[B. Yanchitsky]

yxl at email.jlu.edu.cn wrote:
> Dear users,
>  I have a question for a long time. when we make a supercell simulating an impurity or a surface ,we treat the supercell as
unit cell.So I think we should'nt accept the struct generated by nn or sgroup,But it isn't true. Can you explain it for me.I'll
be appreciated for you.
> yours sincerecely,
> hongxia
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>


Your struct file (unit cell, supercell) must correspond
to one of the 230 space groups, the word "correspond" means
correct lattice vectors and their orientations, correct atom multiplicity, etc. Due to some
specific features of the wien package, there are cases when some symmetry restrictions
became somewhat easy, so you might proceed further even if sgroup generates warnings. But one
should understand clearly why nn complains, why sgroup complains,
and why this can be ignored.
As an example, the structure is cubic, atom "A" at
(0,0,0) atom "B" at (1/2,1/2,1/2). We give a small displacement to atom "B", so the
new coordinates of "B" are "(1/2+x,1/2+x,1/2+x). Real symmetry now is rhombohedral, sgroup
complains about wrong Bravais lattice, but you might ignore this and keep
"cubic" setting, because program symmetry is able to detect all symmetry operations.
Actually i never use the tricks like that, because it is possible to get
problems with optimisation of the unit cell.
For the example above, for rhombohedral we have alpha=beta=gamma, but how to change the
angles and keep "cubic" setting? This is impossible, symmetry now can't determine
symmetry operations, to perform complete optimisation of the unit cell, we need true
rhombohedral setting.

Best,
Bogdan