[Wien] Diamond lattice

Peter Blaha pblaha at zeus.theochem.tuwien.ac.at
Mon Jan 5 14:52:33 CET 2004

> > In the case of Diamond you need t put the atom at 1/4 1/4 1/4. Only one
> > atom.
> > Below is a struct file which works.
> Thank you, that works. Do you happen to know why this has to be this way? When
> using a drawing program, I can construct the lattice using the spacegroup and
> one atom at (0,0,0), a 8a-Wyckoff position. The lattice composed by WIEN is
> the same, although the cell looks rather awkwardly due to the 1/4-shift.

This spacegroup #227 has two different settings of origin. One with
(0,0,0) at 16c, pointgroup -3m, i.e. it has inversion symmetry at the origin
of the cell. In this setting the two C atoms are at position 8a (1/8,1/8,1/8)

The "more common" setting of this spacegroup has the origin (0,0,0) at 8a,
which has pointgroup -43m, i.e. it DOES NOT have inversion. This is the setting
in which the diamond structure is usually given (since a C atom is at
the origin of the cube) and people like this drawing.

However, in this setting all matrix elements in a calculation would be
complex numbers and the calculation would take 4x as long and need 2x memory.

Thus WIEN2k REQUIRES to use a setting where inversion is at the origin of
the cell.

What do you learn from this: The name of the spacegroup + some coordinates
of an atom are sometimes NOT sufficient to describe a structure. You must
either also specify which "setting" of the spacegroup was chosen or
specify which Wyckoff position was given (and from this you know which setting
was used).

Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/

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