From 62a501ebda59547bb3483fb2511df6266190e4e0 Mon Sep 17 00:00:00 2001
From: Axel Kohlmeyer <akohlmey@gmail.com>
Date: Sun, 9 Aug 2020 01:27:14 -0400
Subject: [PATCH] store cumulative energy change in restart file

---
 doc/src/fix_temp_berendsen.rst |  6 +++++-
 src/fix_temp_berendsen.cpp     | 29 +++++++++++++++++++++++++++++
 src/fix_temp_berendsen.h       |  2 ++
 3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/doc/src/fix_temp_berendsen.rst b/doc/src/fix_temp_berendsen.rst
index 2774075827..e837a21847 100644
--- a/doc/src/fix_temp_berendsen.rst
+++ b/doc/src/fix_temp_berendsen.rst
@@ -119,7 +119,11 @@ thermal degrees of freedom, and the bias is added back in.
 
 **Restart, fix_modify, output, run start/stop, minimize info:**
 
-No information about this fix is written to :doc:`binary restart files <restart>`.
+This fix writes the cumulative global energy change to
+:doc:`binary restart files <restart>`.  See the
+:doc:`read_restart <read_restart>` command for info on how to
+re-specify a fix in an input script that reads a restart file,
+so that the fix continues in an uninterrupted fashion.
 
 The :doc:`fix_modify <fix_modify>` *temp* option is supported by this
 fix.  You can use it to assign a temperature :doc:`compute <compute>`
diff --git a/src/fix_temp_berendsen.cpp b/src/fix_temp_berendsen.cpp
index 9b385bf96c..f2b7940584 100644
--- a/src/fix_temp_berendsen.cpp
+++ b/src/fix_temp_berendsen.cpp
@@ -43,6 +43,7 @@ FixTempBerendsen::FixTempBerendsen(LAMMPS *lmp, int narg, char **arg) :
 
   // Berendsen thermostat should be applied every step
 
+  restart_global = 1;
   dynamic_group_allow = 1;
   nevery = 1;
   scalar_flag = 1;
@@ -241,6 +242,34 @@ double FixTempBerendsen::compute_scalar()
   return energy;
 }
 
+/* ----------------------------------------------------------------------
+   pack entire state of Fix into one write
+------------------------------------------------------------------------- */
+
+void FixTempBerendsen::write_restart(FILE *fp)
+{
+  int n = 0;
+  double list[1];
+  list[n++] = energy;
+
+  if (comm->me == 0) {
+    int size = n * sizeof(double);
+    fwrite(&size,sizeof(int),1,fp);
+    fwrite(list,sizeof(double),n,fp);
+  }
+}
+
+/* ----------------------------------------------------------------------
+   use state info from restart file to restart the Fix
+------------------------------------------------------------------------- */
+
+void FixTempBerendsen::restart(char *buf)
+{
+  int n = 0;
+  double *list = (double *) buf;
+
+  energy = list[n++];
+}
 /* ----------------------------------------------------------------------
    extract thermostat properties
 ------------------------------------------------------------------------- */
diff --git a/src/fix_temp_berendsen.h b/src/fix_temp_berendsen.h
index ffc0ca5def..5657b0fd97 100644
--- a/src/fix_temp_berendsen.h
+++ b/src/fix_temp_berendsen.h
@@ -34,6 +34,8 @@ class FixTempBerendsen : public Fix {
   int modify_param(int, char **);
   void reset_target(double);
   double compute_scalar();
+  void write_restart(FILE *);
+  void restart(char *buf);
   virtual void *extract(const char *, int &);
 
  private: