diff --git a/src/MOLECULE/dump_bond.cpp b/src/MOLECULE/dump_bond.cpp
index ef3e55bfac..4ce8e907cb 100644
--- a/src/MOLECULE/dump_bond.cpp
+++ b/src/MOLECULE/dump_bond.cpp
@@ -34,10 +34,6 @@ DumpBond::DumpBond(LAMMPS *lmp, int narg, char **arg) : Dump(lmp, narg, arg)
   int n = strlen(str) + 1;
   format_default = new char[n];
   strcpy(format_default,str);
-
-  // one-time file open
-
-  if (multifile == 0) openfile();
 }
 
 /* ---------------------------------------------------------------------- */
@@ -53,6 +49,10 @@ void DumpBond::init()
   format = new char[n];
   strcpy(format,str);
   strcat(format,"\n");
+
+  // open single file, one time only
+
+  if (multifile == 0) openfile();
 }
 
 /* ---------------------------------------------------------------------- */
diff --git a/src/dump.cpp b/src/dump.cpp
index 3f0b6c3ed6..e0d2271685 100644
--- a/src/dump.cpp
+++ b/src/dump.cpp
@@ -53,6 +53,7 @@ Dump::Dump(LAMMPS *lmp, int narg, char **arg) : Pointers(lmp)
   format_user = NULL;
 
   sort_flag = 0;
+  append_flag = 0;
 
   maxbuf = 0;
   buf = NULL;
@@ -65,6 +66,7 @@ Dump::Dump(LAMMPS *lmp, int narg, char **arg) : Pointers(lmp)
   //   else if ends in .gz = gzipped text file
   //   else ASCII text file
 
+  singlefile_opened = 0;
   compressed = 0;
   binary = 0;
   multifile = 0;
@@ -233,7 +235,22 @@ void Dump::modify_params(int narg, char **arg)
 
   int iarg = 0;
   while (iarg < narg) {
-    if (strcmp(arg[iarg],"format") == 0) {
+    if (strcmp(arg[iarg],"append") == 0) {
+      if (iarg+2 > narg) error->all("Illegal dump_modify command");
+      if (strcmp(arg[iarg+1],"yes") == 0) append_flag = 1;
+      else if (strcmp(arg[iarg+1],"no") == 0) append_flag = 0;
+      else error->all("Illegal dump_modify command");
+      iarg += 2;
+    } else if (strcmp(arg[iarg],"every") == 0) {
+      if (iarg+2 > narg) error->all("Illegal dump_modify command");
+      int n = atoi(arg[iarg+1]);
+      if (n <= 0) error->all("Illegal dump_modify command");
+      int idump;
+      for (idump = 0; idump < output->ndump; idump++)
+	if (strcmp(id,output->dump[idump]->id) == 0) break;
+      output->dump_every[idump] = n;
+      iarg += 2;
+    } else if (strcmp(arg[iarg],"format") == 0) {
       if (iarg+2 > narg) error->all("Illegal dump_modify command");
       delete [] format_user;
       format_user = NULL;
@@ -249,15 +266,6 @@ void Dump::modify_params(int narg, char **arg)
       else if (strcmp(arg[iarg+1],"no") == 0) flush_flag = 0;
       else error->all("Illegal dump_modify command");
       iarg += 2;
-    } else if (strcmp(arg[iarg],"every") == 0) {
-      if (iarg+2 > narg) error->all("Illegal dump_modify command");
-      int n = atoi(arg[iarg+1]);
-      if (n <= 0) error->all("Illegal dump_modify command");
-      int idump;
-      for (idump = 0; idump < output->ndump; idump++)
-	if (strcmp(id,output->dump[idump]->id) == 0) break;
-      output->dump_every[idump] = n;
-      iarg += 2;
     } else if (strcmp(arg[iarg],"sort") == 0) {
       if (iarg+2 > narg) error->all("Illegal dump_modify command");
       if (strcmp(arg[iarg+1],"yes") == 0) sort_flag = 1;
@@ -290,6 +298,11 @@ double Dump::memory_usage()
 
 void Dump::openfile()
 {
+  // single file, already opened, so just return
+
+  if (singlefile_opened) return;
+  if (multifile == 0) singlefile_opened = 1;
+
   // if one file per timestep, replace '*' with current timestep
 
   char *filecurrent;
@@ -315,6 +328,8 @@ void Dump::openfile()
 #endif
     } else if (binary) {
       fp = fopen(filecurrent,"wb");
+    } else if (append_flag) {
+      fp = fopen(filecurrent,"a");
     } else {
       fp = fopen(filecurrent,"w");
     }
diff --git a/src/dump.h b/src/dump.h
index 8effa149ad..5823fc8a4e 100644
--- a/src/dump.h
+++ b/src/dump.h
@@ -35,6 +35,8 @@ class Dump : protected Pointers {
   int header_flag;           // 0 = item, 2 = xyz
   int flush_flag;            // 0 if no flush, 1 if flush every dump
   int sort_flag;             // 1 if write in sorted order, 0 if not
+  int append_flag;           // 1 if open file in append mode, 0 if not
+  int singlefile_opened;     // 1 = one big file, already opened, else 0
 
   char *format_default;      // default format string
   char *format_user;         // format string set by user
diff --git a/src/dump_atom.cpp b/src/dump_atom.cpp
index d4700ce86a..8635546ef3 100644
--- a/src/dump_atom.cpp
+++ b/src/dump_atom.cpp
@@ -30,10 +30,6 @@ DumpAtom::DumpAtom(LAMMPS *lmp, int narg, char **arg) : Dump(lmp, narg, arg)
   scale_flag = 1;
   image_flag = 0;
   format_default = NULL;
-
-  // one-time file open
-
-  if (multifile == 0) openfile();
 }
 
 /* ---------------------------------------------------------------------- */
@@ -99,6 +95,10 @@ void DumpAtom::init()
   if (binary) write_choice = &DumpAtom::write_binary;
   else if (image_flag == 0) write_choice = &DumpAtom::write_noimage;
   else if (image_flag == 1) write_choice = &DumpAtom::write_image;
+
+  // open single file, one time only
+
+  if (multifile == 0) openfile();
 }
 
 /* ---------------------------------------------------------------------- */
diff --git a/src/dump_custom.cpp b/src/dump_custom.cpp
index 7501713264..81e1331297 100644
--- a/src/dump_custom.cpp
+++ b/src/dump_custom.cpp
@@ -112,10 +112,6 @@ DumpCustom::DumpCustom(LAMMPS *lmp, int narg, char **arg) :
     strcat(columns,arg[iarg]);
     strcat(columns," ");
   }
-
-  // one-time file open
-
-  if (multifile == 0) openfile();
 }
 
 /* ---------------------------------------------------------------------- */
@@ -219,6 +215,10 @@ void DumpCustom::init()
     if (ivariable < 0) error->all("Could not find dump custom variable name");
     variable[i] = ivariable;
   }
+
+  // open single file, one time only
+
+  if (multifile == 0) openfile();
 }
 
 /* ---------------------------------------------------------------------- */
diff --git a/src/dump_xyz.cpp b/src/dump_xyz.cpp
index 0656843f5c..5aaca3c396 100644
--- a/src/dump_xyz.cpp
+++ b/src/dump_xyz.cpp
@@ -45,9 +45,6 @@ DumpXYZ::DumpXYZ(LAMMPS *lmp, int narg, char **arg) : Dump(lmp, narg, arg)
     coords = (float *) memory->smalloc(3*natoms*sizeof(float),"dump:coords");
   }
 
-  // one-time file open
-
-  if (multifile == 0) openfile();
   ntotal = 0;
 }
 
@@ -74,6 +71,10 @@ void DumpXYZ::init()
   format = new char[n];
   strcpy(format,str);
   strcat(format,"\n");
+
+  // open single file, one time only
+
+  if (multifile == 0) openfile();
 }
 
 /* ----------------------------------------------------------------------