[Pw_forum] ibrav and celldm(4)

[Paolo Giannozzi]

On Jun 25, 2011, at 23:39 , Abolore Musari wrote:

> alpha equal beta equal 90 degree

beta = 90 ...

> beta was gotten from reitveld refinement to be 113.81

...or beta = 113.81?

> i dont know if my celldm(4) which is equal to cos(ab) should
> be cos90 or cos113.81 or tell me the appropriate way to get
> celldm(4)=cos(ab).

the monoclinic (P? ibrav=12?) lattice can be described with
unique (i.e. orthogonal to a) axis c, b axis at angle gamma /= 90
with a; or with unique axis b, c axis at angle beta /= 90 with a.
QE uses the former description; apparently you have found
the latter description in the literature, so you have to either
convert the latter into the former, or write down explicitly the
crystal axis, or to use the latest svn version that allows for
the latter description as well (ibrav=-12)

P.
---
Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222