Fix issue #178

CHANGELOG.md

@@ -10,6 +10,7 @@ Changelog
 * Persist alchemical ion identity across restarts so the same molecule is reused regardless of GCMC state [#172](https://github.com/OpenBioSim/somd2/pull/172).
 * Use perisistent `ThreadPoolExector` objects within the main replica exchange dynamics block [#175](https://github.com/OpenBioSim/somd2/pull/175).
 * Allow `oversubscription_factor` to change on restart [#177](https://github.com/OpenBioSim/somd2/pull/177).
+* Restrict energy component decomposition to force groups that are used for integration [#180](https://github.com/OpenBioSim/somd2/pull/180).
 
 [2026.1.0](https://github.com/openbiosim/somd2/compare/2025.1.0...2026.1.0) - Jun 2026
 --------------------------------------------------------------------------------------

src/somd2/runner/_base.py

@@ -615,6 +615,9 @@ def __init__(self, system, config):
         # used to skip duplicate rows on restart.
         self._last_ec_time = {}
 
+        # Per-window cache of the integrator's integration force groups bitmask.
+        self._integration_groups = {}
+
         # Store the current system as a reference.
         self._reference_system = self._system.clone()
 
@@ -2514,10 +2517,19 @@ def _save_energy_components(self, index, context, time_ns):
         if time_ns <= self._last_ec_time[index]:
             return
 
+        if index not in self._integration_groups:
+            self._integration_groups[index] = (
+                context.getIntegrator().getIntegrationForceGroups()
+            )
+        integration_groups = self._integration_groups[index]
+
         # Use the named force groups already assigned by sire_to_openmm_system,
         # sorted alphabetically for a consistent column order across runs.
+        # Skip any group not actually used for integration.
         energies = {}
         for name, grp in sorted(context._force_group_map.items()):
+            if not integration_groups & (1 << grp):
+                continue
             state = context.getState(getEnergy=True, groups=(1 << grp))
             energies[name] = state.getPotentialEnergy().value_in_unit(
                 openmm.unit.kilocalories_per_mole