[Pw_forum] monoclinic cell definition question

[Jonas Baltrusaitis]

To reiterate on my previous question: while a,b,c parameters can be transferred to pwscf unambiguously, angles ab, bc and ac are not ac clear. Are they equivalent to alfa beta and gamma, respectively, according to Wyckoff definition?

Jonas


--- On Mon, 5/11/09, Jonas Baltrusaitis <jasius_1 at yahoo.com> wrote:

> From: Jonas Baltrusaitis <jasius_1 at yahoo.com>
> Subject: monoclinic cell definition question
> To: "PWSCF Forum" <pw_forum at pwscf.org>
> Date: Monday, May 11, 2009, 6:14 PM
> I found discrepancies in how monoclinic cell is defined in
> the manual and how it's defined in experimental
> crystalography files:
> 
> Namely, 
> 
> For P lattices: the special axis (c) is the z-axis, one
> basal-plane
> vector (a) is along x, the other basal-plane vector (b) is
> at angle
> gamma for monoclinic. While it is not clear angle to which
> axis, later on gamma is defined as an angle between a and b.
> the problem is that ab is 90 degree angle, not a unique
> angle, if I visualize the structure with cell axes.
> 
> Furthermore, when I open any crystalography file in
> Mercury, I get for monoclinic cell
> 
> alpha 90.00 betta 115.6670(10) gamma 90.00 so gamma is
> again not a unique angle but rather 90 degree angle. I
> assume this is not what needs to be entered for monoclinic
> lattice in celldm(4), but rather a unique angle which in
> both cases is between a and c, so it has nothing to do with
> b as defined above
> 
> Could anybody help out?
> 
> Jonas Baltrusaitis
> University of Iowa