[Wien] unit in optimization results

[Gerhard Fecher]

You just figured out that the volume is that of the primitive and not of the cubic cell, and that was always the case.
Uually the primitive cell for fcc or bcc lattices is drawn in every textbook. You may also consult the webpage of the bilbao crrystallography server, to see all primitive cells and Brillouin zones.

-----Ursprüngliche Nachricht-----
Von: wien-bounces at zeus.theochem.tuwien.ac.at im Auftrag von Shu Miao
Gesendet: So 01.10.2006 09:40
An: wien at zeus.theochem.tuwien.ac.at
Betreff: [Wien] unit in optimization results
 
Dear Wien2k users
I'm reporting a problem in the optimization results. The optimize program
in the new version will automatically fit the optimization results to find
out the equilibrium volume. According to previous posts in the mailing
list, this should be the volume in bohr^3 and the lattice parameter of a
cubic cell can be obtained by taking the cubic root. However, this seems
not true. I tried with MgO, a cubic material with space group Fm-3m. The
a0 is about 7.988 bohr. The optimized volume (V0) is 130.1788. This gives
a cubic root 5.068 bohr. This is far from what it should be. But this is
not wrong. I find out that the V0 is not the volume of the unit cell, but
just the volume of a pair of Mg/O. Since this is a FCC structure, it
contains 4 pairs of Mg/O. If we multiply 130.1788 by a factor 4, the cubic
root will give correct lattice parameter.
Has anyone noted this problem? Is it also true for other systems? If so,
it can be a lot of trouble to get the optimized lattice parameters in a
low symmetry, multi-atom structure.

Thanks

Shu Miao
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