Re: [Spglib-users] Tolerances differ in get_int and get_sym?

[Atsushi T. <atz...@gm...>]

Hello,

I guess this is not a bug.

spg_get_symmetry just searches symmetry operations by trying possible
operations and checking overlapping of atoms within tolerance
distance.
spg_get_international uses symmetry operations found by
spg_get_symmetry as the starting point. Then it compares with the
database of space groups. In this database matching, tolerance is also
used. The ways that tolerance used for symmetry operation search and
database matching are not exactly same. So they can make different
results in crystal symmetry sense.

You can also obtain symmetry operations from spg_get_dataset, which
are consistent with the space group that is found, because the
symmetry operations in spg_get_dataset are those recovered from the
space group database.  So I recommend to use this and I consider this
set of symmetry operations used as return values of spg_get_symmetry
in the future. There is another problem of spg_get_symmetry. In some
case, the symmetry operations obtained by spg_get_symmetry don't form
group (when the obtained symmetry operations are considered as coset
representatives in the terminology of group theory).

Togo

> I am, in general, very impressed with spglib. It does a difficult and
> tedious task well.
>
> However, one point confuses me. It seems that the symmetry operations
> returned by spg_get_symmetry are not always consistent with the
> international symbol found by spg_get_international when the same
> tolerance is used for both. The attached example prints
>
> Symprec=1e-05, int symbol is Cmc2_1
> Symprec=2e-05, int symbol is Cmc2_1
> Symprec=0.0001, int symbol is Cmc2_1
>
> Symprec=1e-05, 4 symmetry operations
> Symprec=2e-05, 4 symmetry operations
> Symprec=0.0001, 8 symmetry operations
>
> whereas I would expect Cmc2_1 to have 8 symmetry operations always.
>
> Am I confused, or is this a slight bug in spglib? (I am using version
> 1.3 from Sourceforge.)
>
> Regards,
>
> Michael
>
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-- 
Atsushi Togo
http://atztogo.users.sourceforge.net/
atz...@gm...