Fixes Arkane QM corrections file path

Updates the Arkane quantum corrections file path resolution.
The code now correctly identifies the location of the `data.py`
file containing QM corrections by checking multiple possible paths.
Raises an InputError if the file cannot be located, guiding the user
to set the RMG_DB_PATH correctly. This prevents errors when running
Arkane calculations due to missing quantum corrections data.

This ensures that the Arkane model chemistry and BAC checks
correctly identify and utilize the quantum corrections data.

Author: calvinp0

arc/statmech/arkane.py

@@ -692,8 +692,16 @@ def _get_qm_corrections_files() -> List[str]:
     """
     candidates = [
         os.path.join(RMG_DB_PATH, 'input', 'quantum_corrections', 'data.py'),
+        os.path.join(RMG_DB_PATH, 'quantum_corrections', 'data.py'),
     ]
-    return [path for path in candidates if os.path.isfile(path)]
+    existing = [path for path in candidates if os.path.isfile(path)]
+    if not existing:
+        raise InputError(
+            "Could not locate Arkane quantum corrections data.py. "
+            f"Checked: {', '.join(candidates)}. "
+            "Please set RMG_DB_PATH to a valid RMG database."
+        )
+    return existing
 
 
 def _normalize_method(method: str) -> str:
@@ -950,9 +958,7 @@ def get_arkane_model_chemistry(sp_level: 'Level',
     Returns:
         Optional[str]: Arkane-compatible model chemistry string.
     """
-    qm_corr_file = os.path.join(RMG_DB_PATH, 'input', 'quantum_corrections', 'data.py')
-    if not os.path.isfile(qm_corr_file):
-        qm_corr_file = os.path.join(RMG_DB_PATH, 'quantum_corrections', 'data.py')
+    qm_corr_files = _get_qm_corrections_files()
 
     atom_energies_start = "atom_energies = {"
     atom_energies_end = "pbac = {"
@@ -1081,9 +1087,7 @@ def check_arkane_bacs(sp_level: 'Level',
       - basis (normalized)
     and picking the latest year where multiple exist.
     """
-    qm_corr_file = os.path.join(RMG_DB_PATH, 'input', 'quantum_corrections', 'data.py')
-    if not os.path.isfile(qm_corr_file):
-        qm_corr_file = os.path.join(RMG_DB_PATH, 'quantum_corrections', 'data.py')
+    qm_corr_files = _get_qm_corrections_files()
 
     atom_energies_start = "atom_energies = {"
     atom_energies_end = "pbac = {"