From 626e3818a325f07add061fed5287ced77697a58a Mon Sep 17 00:00:00 2001
From: Berk Hess <hess@kth.se>
Date: Tue, 30 Oct 2018 09:46:53 +0100
Subject: [PATCH] Work around expanded ensemble issues

Two bugs could cause expanded ensemble sampling to use
outdated or zero energies. In these cases mdrun now modifies
nstcalcenergy to 1 to avoid these bugs.

Note: This change should not be merged upstream, since there
is a proper fix for release-2019.

Refs #2714
Refs #2718

Change-Id: I79be9c5da55eaebb857bac6a98e6671720532e0e
---
 docs/release-notes/2018/2018.4.rst | 11 +++++++++++
 src/programs/mdrun/md.cpp          | 14 ++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/docs/release-notes/2018/2018.4.rst b/docs/release-notes/2018/2018.4.rst
index e754d0b84a..14fbcad41d 100644
--- a/docs/release-notes/2018/2018.4.rst
+++ b/docs/release-notes/2018/2018.4.rst
@@ -37,6 +37,17 @@ contribution was missing from the foreign Hamiltonian values.
 
 :issue: `2703`
 
+Work around bugs with expanded ensemble runs
+""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+
+With expanded ensemble runs, the energies would be outdated or zero
+with the velocity Verlet integrator with nstcalcenergy>1 or with
+other integrators when nstexpanded was not a multiple of nstcalcenergy.
+In these cases mdrun now sets nstcalcenergy to 1.
+
+:issue: `2714`
+:issue: `2718`
+
 Fixes for ``gmx`` tools
 ^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/src/programs/mdrun/md.cpp b/src/programs/mdrun/md.cpp
index 1695e0b72d..de5a7031ff 100644
--- a/src/programs/mdrun/md.cpp
+++ b/src/programs/mdrun/md.cpp
@@ -125,6 +125,7 @@
 #include "gromacs/utility/logger.h"
 #include "gromacs/utility/real.h"
 #include "gromacs/utility/smalloc.h"
+#include "gromacs/utility/stringutil.h"
 
 #include "deform.h"
 #include "membed.h"
@@ -315,6 +316,19 @@ double gmx::do_md(FILE *fplog, t_commrec *cr, const gmx::MDLogger &mdlog,
                   gmx_membed_t *membed,
                   gmx_walltime_accounting_t walltime_accounting)
 {
+    /* Workaround for 2 bugs in release-2018.
+     * NOTE: Proper fix is in release-2019, do not merge this change there.
+     */
+    if (ir->bExpanded && (EI_VV(ir->eI) ||
+                          ir->expandedvals->nstexpanded % ir->nstcalcenergy != 0))
+    {
+        ir->nstcalcenergy = 1;
+        std::string note =
+            gmx::formatString("NOTE: There are issues with expanded ensemble and certain combination of nstexpanded and nstcalcenergy, setting nstcalcenergy to %d",
+                              ir->nstcalcenergy);
+        GMX_LOG(mdlog.warning).asParagraph().appendText(note);
+    }
+
     gmx_mdoutf_t    outf = nullptr;
     gmx_int64_t     step, step_rel;
     double          elapsed_time;
-- 
2.11.4.GIT