Bug: Symmetry group of caffeine

[joerivan]

ZeroDivisionError when analysing caffeine molecule classified as Cs

I encountered a crash when running pyrrhotite on a caffeine molecule from an .xyz file. I have attached/uploaded the caffeine.xyz file to this issue.

Observed behaviour

The molecule appears to be classified as point group Cs, but the program crashes while labelling reflection planes:

line 123, in __init__ 
    self._label_symmetry_operations()
ine 1087, in _label_symmetry_operations 
    self._label_reflection_planes()
line 1164, in _label_reflection_planes 
    self._label_reflection_planes_cyclic_dihedral()
line 1205, in _label_reflection_planes_cyclic_dihedral 
    divisor = 2.0 * np.pi / pg_label.order
ZeroDivisionError: float division by zero

Expected behaviour

The program should complete normally and report the Cs symmetry operation, for example:

Point group : Cs
Rotor class : AsymmetricTop
Operations  : 1 found
  σ

Possible cause

It looks like Cs is being routed through the cyclic/dihedral reflection-plane labelling code. However, for Cs, the point-group order appears to be 0, so this line fails:

divisor = 2.0 * np.pi / pg_label.order

Since Cs only has a single mirror plane, there may not be a meaningful σh/σv/σd distinction to compute. The mirror plane could simply remain labelled as σ.

Proposed fix

A minimal fix would be to special-case point groups with order == 0 at the start of _label_reflection_planes_cyclic_dihedral():

def _label_reflection_planes_cyclic_dihedral(self) -> None:
     """Label σh, σv, σd, σv' for cyclic and dihedral groups."""
     pg_label = self._point_group.label
 
+    # Special low-symmetry groups such as Cs do not have a finite
+    # principal rotation order.  Their mirror operation should remain
+    # labelled as plain σ; there is no σh/σv/σd distinction to compute.
+    if pg_label.order == 0:
+        return
+
     divisor = 2.0 * np.pi / pg_label.order

Alternatively, Cs could be excluded earlier from the cyclic/dihedral reflection-plane labelling path and handled explicitly as a low-symmetry special case.

caffeine.zip