diff --git a/src/USER-COLVARS/fix_colvars.cpp b/src/USER-COLVARS/fix_colvars.cpp
index 2f8ca857cb..9adde33f6d 100644
--- a/src/USER-COLVARS/fix_colvars.cpp
+++ b/src/USER-COLVARS/fix_colvars.cpp
@@ -300,7 +300,7 @@ FixColvars::FixColvars(LAMMPS *lmp, int narg, char **arg) :
   if (atom->rmass_flag)
     error->all(FLERR,"Cannot use fix colvars for atoms with rmass attribute");
 
-  if (instances)
+  if (instances > 0)
     error->all(FLERR,"Only one fix colvars can be active at a time");
   ++instances;
 
@@ -313,6 +313,7 @@ FixColvars::FixColvars(LAMMPS *lmp, int narg, char **arg) :
 
   conf_file = strdup(arg[3]);
   rng_seed = 1966;
+  unwrap_flag = 0;
 
   inp_name = NULL;
   out_name = NULL;
@@ -327,6 +328,14 @@ FixColvars::FixColvars(LAMMPS *lmp, int narg, char **arg) :
       out_name = strdup(arg[argsdone+1]);
     } else if (0 == strcmp(arg[argsdone], "seed")) {
       rng_seed = atoi(arg[argsdone+1]);
+    } else if (0 == strcmp(arg[argsdone], "unwrap")) {
+      if (0 == strcmp(arg[argsdone+1], "yes")) {
+        unwrap_flag = 1;
+      } else if (0 == strcmp(arg[argsdone+1], "no")) {
+        unwrap_flag = 0;
+      } else {
+        error->all(FLERR,"Incorrect fix colvars unwrap flag");
+      }
     } else if (0 == strcmp(arg[argsdone], "tstat")) {
       tmp_name = strdup(arg[argsdone+1]);
     } else {
@@ -538,6 +547,14 @@ void FixColvars::init()
   memory->create(comm_buf,nmax,"colvars:comm_buf");
 
   const double * const * const x = atom->x;
+  const tagint * const image = atom->image;
+
+  const double xprd = domain->xprd;
+  const double yprd = domain->yprd;
+  const double zprd = domain->zprd;
+  const double xy = domain->xy;
+  const double xz = domain->xz;
+  const double yz = domain->yz;
 
   if (me == 0) {
 
@@ -549,12 +566,24 @@ void FixColvars::init()
     for (i=0; i<num_coords; ++i) {
       const int k = atom->map(taglist[i]);
       if ((k >= 0) && (k < nlocal)) {
+
         of[i].tag  = cd[i].tag  = tag[k];
         of[i].type = cd[i].type = type[k];
-        cd[i].x = x[k][0];
-        cd[i].y = x[k][1];
-        cd[i].z = x[k][2];
         of[i].x = of[i].y = of[i].z = 0.0;
+
+        if (unwrap_flag) {
+          const int ix = (image[k] & IMGMASK) - IMGMAX;
+          const int iy = (image[k] >> IMGBITS & IMGMASK) - IMGMAX;
+          const int iz = (image[k] >> IMG2BITS) - IMGMAX;
+
+          cd[i].x = x[k][0] + ix * xprd + iy * xy + iz * xz;
+          cd[i].y = x[k][1] + iy * yprd + iz * yz;
+          cd[i].z = x[k][2] + iz * zprd;
+        } else {
+          cd[i].x = x[k][0];
+          cd[i].y = x[k][1];
+          cd[i].z = x[k][2];
+        }
       }
     }
 
@@ -588,11 +617,24 @@ void FixColvars::init()
     for (i=0; i<num_coords; ++i) {
       const int k = atom->map(taglist[i]);
       if ((k >= 0) && (k < nlocal)) {
+
         comm_buf[nme].tag  = tag[k];
         comm_buf[nme].type = type[k];
-        comm_buf[nme].x    = x[k][0];
-        comm_buf[nme].y    = x[k][1];
-        comm_buf[nme].z    = x[k][2];
+
+        if (unwrap_flag) {
+          const int ix = (image[k] & IMGMASK) - IMGMAX;
+          const int iy = (image[k] >> IMGBITS & IMGMASK) - IMGMAX;
+          const int iz = (image[k] >> IMG2BITS) - IMGMAX;
+
+          comm_buf[nme].x = x[k][0] + ix * xprd + iy * xy + iz * xz;
+          comm_buf[nme].y = x[k][1] + iy * yprd + iz * yz;
+          comm_buf[nme].z = x[k][2] + iz * zprd;
+        } else {
+          comm_buf[nme].x = x[k][0];
+          comm_buf[nme].y = x[k][1];
+          comm_buf[nme].z = x[k][2];
+        }
+
         ++nme;
       }
     }
@@ -643,6 +685,14 @@ void FixColvars::post_force(int vflag)
   const int * const tag = atom->tag;
   const double * const * const x = atom->x;
   double * const * const f = atom->f;
+  const tagint * const image = atom->image;
+
+  const double xprd = domain->xprd;
+  const double yprd = domain->yprd;
+  const double zprd = domain->zprd;
+  const double xy = domain->xy;
+  const double xz = domain->xz;
+  const double yz = domain->yz;
   const int nlocal = atom->nlocal;
 
   /* check and potentially grow local communication buffers. */
@@ -671,9 +721,20 @@ void FixColvars::post_force(int vflag)
     for (i=0; i<num_coords; ++i) {
       const int k = atom->map(taglist[i]);
       if ((k >= 0) && (k < nlocal)) {
-        cd[i].x = x[k][0];
-        cd[i].y = x[k][1];
-        cd[i].z = x[k][2];
+
+        if (unwrap_flag) {
+          const int ix = (image[k] & IMGMASK) - IMGMAX;
+          const int iy = (image[k] >> IMGBITS & IMGMASK) - IMGMAX;
+          const int iz = (image[k] >> IMG2BITS) - IMGMAX;
+
+          cd[i].x = x[k][0] + ix * xprd + iy * xy + iz * xz;
+          cd[i].y = x[k][1] + iy * yprd + iz * yz;
+          cd[i].z = x[k][2] + iz * zprd;
+        } else {
+          cd[i].x = x[k][0];
+          cd[i].y = x[k][1];
+          cd[i].z = x[k][2];
+        }
       }
     }
 
@@ -703,9 +764,21 @@ void FixColvars::post_force(int vflag)
       const int k = atom->map(taglist[i]);
       if ((k >= 0) && (k < nlocal)) {
         comm_buf[nme].tag = tag[k];
-        comm_buf[nme].x = x[k][0];
-        comm_buf[nme].y = x[k][1];
-        comm_buf[nme].z = x[k][2];
+
+        if (unwrap_flag) {
+          const int ix = (image[k] & IMGMASK) - IMGMAX;
+          const int iy = (image[k] >> IMGBITS & IMGMASK) - IMGMAX;
+          const int iz = (image[k] >> IMG2BITS) - IMGMAX;
+
+          comm_buf[nme].x = x[k][0] + ix * xprd + iy * xy + iz * xz;
+          comm_buf[nme].y = x[k][1] + iy * yprd + iz * yz;
+          comm_buf[nme].z = x[k][2] + iz * zprd;
+        } else {
+          comm_buf[nme].x = x[k][0];
+          comm_buf[nme].y = x[k][1];
+          comm_buf[nme].z = x[k][2];
+        }
+
         ++nme;
       }
     }
diff --git a/src/USER-COLVARS/fix_colvars.h b/src/USER-COLVARS/fix_colvars.h
index de144edc58..6e1a5f8993 100644
--- a/src/USER-COLVARS/fix_colvars.h
+++ b/src/USER-COLVARS/fix_colvars.h
@@ -80,6 +80,7 @@ class FixColvars : public Fix {
 
   int nlevels_respa;   // flag to determine respa levels.
   int store_forces;    // flag to determine whether to store total forces
+  int unwrap_flag;     // 1 if atom coords are unwrapped, 0 if not
   static  int instances; // count fix instances, since colvars currently
                          // only supports one instance at a time
 };