materialsproject
Group-subgroup relationship bugs in `SpaceGroup` and `PointGroup` class
Python version
3.10
Pymatgen version
master
Operating system version
No response
Current behavior
Hey, two remarks regarding the is_subgroup method of SpaceGroup:
First, the current implementation does not allow isomorphic group-subgroup relationships (same space group type) because of
https://github.com/materialsproject/pymatgen/blob/02341828b94a001b4f3cbdae6b24eaeeb0df7d7d/src/pymatgen/symmetry/groups.py#L537
For example, I would expect the following code to not raise an error:
assert SpaceGroup("P3").is_subgroup(SpaceGroup("P3"))
The if j not in all_groups needs to be removed in the first iteration.
Second, the current implementation disregards klassengleiche group-supergroup relationships where the overall translational symmetry is increased, but not through centering. For example, I would expect this to not raise an error:
assert SpaceGroup("Fm-3m").is_subgroup(SpaceGroup("Pm-3m"))
However, because of https://github.com/materialsproject/pymatgen/blob/02341828b94a001b4f3cbdae6b24eaeeb0df7d7d/src/pymatgen/symmetry/groups.py#L528 this returns False. I assume it is to save resources? I would either remove it completely (safest) or restrict it to certain cases, if possible(?).
Also, I had a look at the uncommented PointGroup subgroup test:
https://github.com/materialsproject/pymatgen/blob/02341828b94a001b4f3cbdae6b24eaeeb0df7d7d/tests/symmetry/test_groups.py#L59
This test is failing because the -3 is defined along (111) in m-3m, but along (001) in -3m (therefore, the matrices are different). Generally, this affects all group-subgroup relationships if the groups are in crystal systems with different crystallographic directions / blickrichtungen. For example, this is also failing,
assert PointGroup("2").is_subgroup(PointGroup("4"))
because the rotation axis is along c in the tetragonal, but is notated along b in the monoclinic system (in this database).
I would specify this in the warning https://github.com/materialsproject/pymatgen/blob/02341828b94a001b4f3cbdae6b24eaeeb0df7d7d/src/pymatgen/symmetry/groups.py#L79
In the future, it would probably be best to have a database similar to the maximal subgroup database of space group types.
If you agree, I can implement and test the SpaceGroup changes.
Expected Behavior
I would expect my code snippets from above to not raise an error.
Minimal example
No response
Relevant files to reproduce this bug
No response
