[Pw_forum] monoclinic cell definition question

[Xenia Filip]

I have the same problem recently and due to a monoclinic cell I realize
that I introduce in a wrong manner the coordinates of atoms and cell
parameters in the input file. I admit that I didn't read carefully the
INPUT_PW.txt file. Due to the discrepancy you talk I think that is not
correct to take (x,y,z), (a,b,c) and (alpha, beta, gamma) from a *.cif
file and put them on the same order in *scf.in file. I talk about the
convention used in *.cif files with somebody which is doing X Ray
Crystalography and if he has right the "b" axis in *.cif I think it
correspond to "c" axis in *.scf.in. That means that it needs to permute
(a,b,c) in (c,a,b), (x,y,z) in (z,x,y), (alpha, beta, gamma) in (gamma,
alpha, beta). I am not expert in doing such calculation, I am interest
just in chemical shift calculations (I'm doing NMR), and unfortunatelly I
didn't succed yet to verify this. I am running now some test but it take
some time (I do not have strong computers).
I'm not sure if my ideea is correct but you can think at this posibility too.

Dr. Xenia Filip
National Institute for R&D of
Isotopic and Molecular Technologies
Cluj 400293, Romania
Tel: 0040 724 321420
Fax: 0040 264 420042
>
> 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
>
>
>
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