# UNDF: UNDF-2026-000000900
--- a/src/lagrangian/molecularDynamics/moleculeCloud/moleculeCloud.C
+++ b/src/lagrangian/molecularDynamics/moleculeCloud/moleculeCloud.C
@@ -274,7 +274,9 @@

     {
-        DynamicList<molecule*> molsToDelete;
+        // CWE-407: molsToDelete was a DynamicList scanned by findIndex() O(N)
+        // for each molecule pair, yielding O(pairs * D) where D = deletions.
+        // Fix: use HashSet<molecule*> for O(1) membership checks.
+        HashSet<molecule*> molsToDeleteSet;

         const labelListList& dil(il_.dil());

@@ -304,13 +306,13 @@
                              || findIndex(pot_.removalOrder(), idJ)
                               < findIndex(pot_.removalOrder(), idI)
                             )
                             {
-                                if (findIndex(molsToDelete, molJ) == -1)
+                                if (!molsToDeleteSet.found(molJ))
                                 {
-                                    molsToDelete.append(molJ);
+                                    molsToDeleteSet.insert(molJ);
                                 }
                             }
-                            else if (findIndex(molsToDelete, molI) == -1)
+                            else if (!molsToDeleteSet.found(molI))
                             {
-                                molsToDelete.append(molI);
+                                molsToDeleteSet.insert(molI);
                             }
                         }
@@ -336,13 +338,13 @@
                          || findIndex(pot_.removalOrder(), idJ)
                           < findIndex(pot_.removalOrder(), idI)
                         )
                         {
-                            if (findIndex(molsToDelete, molJ) == -1)
+                            if (!molsToDeleteSet.found(molJ))
                             {
-                                molsToDelete.append(molJ);
+                                molsToDeleteSet.insert(molJ);
                             }
                         }
-                        else if (findIndex(molsToDelete, molI) == -1)
+                        else if (!molsToDeleteSet.found(molI))
                         {
-                            molsToDelete.append(molI);
+                            molsToDeleteSet.insert(molI);
                         }
                     }
@@ -353,10 +355,10 @@

-        forAll(molsToDelete, mTD)
+        forAllConstIter(HashSet<molecule*>, molsToDeleteSet, iter)
         {
-            deleteParticle(*(molsToDelete[mTD]));
+            deleteParticle(*iter.key());
         }
     }

@@ -419,9 +421,9 @@
                             {
-                                if (findIndex(molsToDelete, molI) == -1)
+                                if (!molsToDeleteSet.found(molI))
                                 {
-                                    molsToDelete.append(molI);
+                                    molsToDeleteSet.insert(molI);
                                 }
                             }
                             else if
@@ -437,9 +439,9 @@
                                 if (molI->origId() > molJ->origId())
                                 {
-                                    if (findIndex(molsToDelete, molI) == -1)
+                                    if (!molsToDeleteSet.found(molI))
                                     {
-                                        molsToDelete.append(molI);
+                                        molsToDeleteSet.insert(molI);
                                     }
                                 }