[Wien] sgroup question

[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
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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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>
> --
>
>                                        P.Blaha
> --------------------------------------------------------------------------
Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna Phone:
+43-1-58801-165300             FAX: +43-1-58801-165982
> Email: blaha at theochem.tuwien.ac.at    WWW:
> http://info.tuwien.ac.at/theochem/
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