added internal fix dummy command to enable more control of fix ordering

src/fix_neigh_history.cpp

@@ -19,6 +19,7 @@
 #include "comm.h"
 #include "neighbor.h"
 #include "neigh_list.h"
+#include "modify.h"
 #include "force.h"
 #include "pair.h"
 #include "memory.h"
@@ -147,6 +148,19 @@ void FixNeighHistory::init()
   if (atom->tag_enable == 0)
     error->all(FLERR,"Neighbor history requires atoms have IDs");
 
+  // this fix must come before any fix which migrates atoms in its pre_exchange()
+  // b/c this fix's pre_exchange() creates per-atom data structure
+  // that data must be current for atom migration to carry it along
+
+  for (int i = 0; i < modify->nfix; i++) {
+    if (modify->fix[i] == this) break;
+    if (modify->fix[i]->pre_exchange_migrate)
+      error->all(FLERR,"Fix neigh_history comes after a fix which "
+                 "migrates atoms in pre_exchange");
+  }
+
+  // setup data struct
+
   allocate_pages();
 }