Sorry Manu, but I think in WIEN2k it is more comlicated:
It is of course true that WYCKPOS offers just two choices for SG 12:
C2/m:unique axis b with a centering of (1/2,1/2,0)
C2/m:unique axis c (0,1/2,1/2)
However, WIEN2k requires for monoclinic centered lattices a centering of
(1/2,0,1/2)
(we call it CXZ-lattice) and the monoclinic angle must be gamma. Thus NONE of
the
settings of WYCKPOS is compatible with WIEN2k.
Instead, when I programmed this a long time ago, my Int. Tables had just two
settings
for SG 12, which are called:
B2/m:unique axis c with a centering of (1/2,0,1/2) (or a proper name of
B112/m)
and
C2/m:unique axis b with a centering of (1/2,1/2,0) (or C12/m1) (this is the
first C2/m of WYCKPOS)
I was lazy and implemented only the first choice and thus WIEN2k supports so
far only
B112/m with xz-centering and angle gamma, or in short: B2/m
Conversion can be made with your nice utility SETSTRU as was indicated by
Gavin Abo
(which also works in case somebody has the I2/m setting of this SG)
or by hand when interchanging all y -- z pos.parameters,angles,latt.params.
Best regards
Peter
Am 17.10.2012 13:52, schrieb J. Manuel Perez-Mato:
As far as I as I remember, sgroup finds and gives the standard space group
corresponding to the structure:
In monoclinic systems there are two alternative standards in the
International Tables: unique axis along b o unique axis along c. For
instance, the Bilbao server normally uses the first one, but in some of
its programs like WYCKPOS, it offers you the two possible standards. It
seems that sgroup uses the second possible standard unique axis c, and
in your case it is only proposing a change to this standard setting.
I do not think there is any error in the output of sgroup and in principle
nothing should be changed, because when announcing the space group:
(C2/m) unique axis c, the (C2/m) here is only used as synonym of space
group Number 12, and it does not mean that the centering translation is a
C centering, i.e. (1/2 1/2 0), which is impossible if the c-axis is
chosen as the monoclinic one.
You can compare with the space group labels and positions in the lists
offered by WYCKPOS in the Bilbao server. You will see that for the
standard (C2/m): unique axis c , the centering translation is in fact
of type A, i.e. (0 1/2 1/2). Therefore, if you inspect the transformed
structure proposed by sgroup, I would expect that you will find that it is
just proposing to interchange the axes a and b, so that your B2/m (unique
axis c) symmetry becomes in fact A2/m(unique axis c), which is the second
standard and it is denoted by (C2/m) unique axis c (surely a very
misleading notation, but it is not the fault of sgroup!).
Regards,
J. Manuel Perez-Mato
Fac. Ciencia y Tecnologia,
Universidad del Pais Vasco, UPV
48080 BILBAO,
Spain
tel. +34 946012473
fax. +34 946013500
*
On Wed, October 17, 2012 10:48 am, Peter Blaha wrote:
I'm not an expert in sgroup, but yes, I believe the printed names of
all
monoclinic spacegroups which have
different names for the unique axis c (vs. b) are not correct and I'll
change them to B or b (instead of C or c)
As far as I know, it does not affect anything else then the comment in
the struct-files generated by sgroup.
Am 16.10.2012 14:04, schrieb Gavin Abo:
Dear Prof. Blaha and developers,
I believe the attached struct file has appropriate parameters for the
12_B2/m spacegroup. After I run x sgroup in w2web in Wien2k 12.1, it
says sgroup found: 12 (C 2/m)
[unique axis c] cell choice 2. I suppose that its most likely
correct,
but was just curious if perhaps the C 2/m is a typo and should be B
2/m in the character array
comnt_B_C2h_sgrp in SRC_sgroup/sgrp_mon.c.
Kind Regards,
Gavin
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