Write GCMC residues after frame is saved.

Author: lohedges

src/somd2/runner/_repex.py

@@ -1265,10 +1265,13 @@ def _run_block(
                 ),
             )
 
-            # Perform a GCMC move and write ghost water residue indices after
-            # dynamics so that the ghost state is temporally consistent with
-            # the saved frame.
             if gcmc_sampler is not None:
+                # Write ghost residues before the GCMC move so the ghost state
+                # is consistent with the saved frame (which is also captured
+                # before the GCMC move).
+                if write_gcmc_ghosts:
+                    gcmc_sampler.write_ghost_residues()
+
                 if is_gcmc:
                     # Push the PyCUDA context on top of the stack.
                     gcmc_sampler.push()
@@ -1280,9 +1283,6 @@ def _run_block(
                     # Remove the PyCUDA context from the stack.
                     gcmc_sampler.pop()
 
-                if write_gcmc_ghosts:
-                    gcmc_sampler.write_ghost_residues()
-
                 # Save the GCMC state.
                 self._dynamics_cache.save_gcmc_state(index)
 

src/somd2/runner/_runner.py

@@ -714,21 +714,22 @@ def generate_lam_vals(lambda_base, increment=0.001):
                                 ),
                             )
 
-                            # Perform a GCMC move.
-                            _logger.info(
-                                f"Performing GCMC move at {_lam_sym} = {lambda_value:.5f}"
-                            )
-                            gcmc_sampler.move(dynamics.context())
-
                             # Update the runtime.
                             runtime += self._config.energy_frequency
 
-                            # If a frame is saved, then we need to save current indices
-                            # of the ghost water residues.
+                            # If a frame is saved, write the ghost residue indices
+                            # before the GCMC move so the ghost state is consistent
+                            # with the saved frame.
                             if save_frames and runtime >= next_frame:
                                 gcmc_sampler.write_ghost_residues()
                                 next_frame += self._config.frame_frequency
 
+                            # Perform a GCMC move.
+                            _logger.info(
+                                f"Performing GCMC move at {_lam_sym} = {lambda_value:.5f}"
+                            )
+                            gcmc_sampler.move(dynamics.context())
+
                     else:
                         dynamics.run(
                             checkpoint_frequency,