From e58743de9046654223d94d8a344ca352aed7e4e0 Mon Sep 17 00:00:00 2001
From: Joseph Barker <j.barker@leeds.ac.uk>
Date: Fri, 27 Feb 2026 12:55:45 +0000
Subject: [PATCH] Fix NVE Spin hang

When running on more than one processor and when atoms belong to different fix groups the nve/spin fix would hang and not integrate.

The issue is caused only in the case that sectoring is true (e.g. MPI with more than one processor) and a sector contains some atoms with the nve/spin fix and some without. There are while loops that become infinite because `i' never gets changed because `mask[i] & groupbit' keeps evaluating as false.

This is fixed by advancing `i' even if we don't need to integrate some spins.
---
 src/SPIN/fix_nve_spin.cpp | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/src/SPIN/fix_nve_spin.cpp b/src/SPIN/fix_nve_spin.cpp
index 8670cd21675..8422cd98653 100644
--- a/src/SPIN/fix_nve_spin.cpp
+++ b/src/SPIN/fix_nve_spin.cpp
@@ -349,22 +349,24 @@ void FixNVESpin::initial_integrate(int /*vflag*/)
       comm->forward_comm();
       int i = stack_foot[j];
       while (i >= 0) {
+        const int next = forward_stacks[i];
         if (mask[i] & groupbit) {
           ComputeInteractionsSpin(i);
           AdvanceSingleSpin(i);
-          i = forward_stacks[i];
         }
+        i = next;
       }
     }
     for (int j = nsectors-1; j >= 0; j--) {     // advance quarter s for nlocal
       comm->forward_comm();
       int i = stack_head[j];
       while (i >= 0) {
+        const int next = backward_stacks[i];
         if (mask[i] & groupbit) {
           ComputeInteractionsSpin(i);
           AdvanceSingleSpin(i);
-          i = backward_stacks[i];
         }
+        i = next;
       }
     }
   } else {                                       // serial seq. update
@@ -402,22 +404,24 @@ void FixNVESpin::initial_integrate(int /*vflag*/)
       comm->forward_comm();
       int i = stack_foot[j];
       while (i >= 0) {
+        const int next = forward_stacks[i];
         if (mask[i] & groupbit) {
           ComputeInteractionsSpin(i);
           AdvanceSingleSpin(i);
-          i = forward_stacks[i];
         }
+        i = next;
       }
     }
     for (int j = nsectors-1; j >= 0; j--) {     // advance quarter s for nlocal
       comm->forward_comm();
       int i = stack_head[j];
       while (i >= 0) {
+        const int next = backward_stacks[i];
         if (mask[i] & groupbit) {
           ComputeInteractionsSpin(i);
           AdvanceSingleSpin(i);
-          i = backward_stacks[i];
         }
+        i = next;
       }
     }
   } else {                                      // serial seq. update