[Wien] PORT, atomic position relaxation

[Stefaan Cottenier]

> For example, if i have a compound belonging to space group 191
> (P6/mmm), i 
> cant relax atomic positions for the special positions.

Indeed, you can't. Look at:

http://www.cryst.ehu.es/cgi-bin/cryst/programs/nph-wp-list?gnum=191

(or at http://www.cryst.ehu.es/cgi-bin/cryst/programs/nph-table?
from=getwp for a list if you want to select a specific space group).

The atoms of your case with space group 191 are at the positions 
indicated in the table by

1 a : >>> Atom 1: 0.0, 0.0, 0.0
2 d : >>> Atom2: 0.3333333, 0.666667, 0.5

In positions 1a and 2d, all coordinates are fixed by symmetry (there 
is no symbol 'x', 'y' or 'z' in the table that can be varied). WIEN 
can only relax atoms at positions with such a free coordinate. In 
almost any case, that is fine: in such high-symmetry positions, there 
is no reason for the atom why it would like to move in one direction 
or another.


> What if i have a compound belonging to space group 62(Pnma) having 2
> non-equivalent atoms in the following positions.

Look at http://www.cryst.ehu.es/cgi-bin/cryst/programs/nph-wp-list?
gnum=62

Your atoms are both at a 4c site (check in case.struct, they both have 
MULT=4, right? That's what the '4' in '4c' means) :

4 c: >> Atom1: 0.177, 0.25, 0.123
4 c: >> Atom2: 0.029, 0.25, 0.603

> So my question is if i deform the unit cell in the principal
> crystallographic 
> direction (just changing a, b or c, not changing the symmetry, the 
> spacegroup remains 62, Pnma). Then i should be able to relax the x
> and z position.

Yes. But there is not need to change a, b, or c. With any choic of abc 
you will be able to relax x an z (unless you have taken the values of 
x and z accidentally such that they are already at the relaxed 
position).

> Does that mean i wont be able to relax the "y" coordinate
> in any  of these cases.

Indeed.

Stefaan