diff --git a/src/REPLICA/prd.cpp b/src/REPLICA/prd.cpp
index e11dae149c..14686f91ac 100644
--- a/src/REPLICA/prd.cpp
+++ b/src/REPLICA/prd.cpp
@@ -431,7 +431,7 @@ void PRD::dephase()
   timer->barrier_start(TIME_LOOP);
 
   for (int i = 0; i < n_dephase; i++) {
-    int seed = static_cast<int> (random_dephase->uniform() * MAXINT32);
+    int seed = static_cast<int> (random_dephase->uniform() * MAXSMALLINT);
     if (seed == 0) seed = 1;
     velocity->create(temp_dephase,seed);
     update->integrate->run(t_dephase);
diff --git a/src/XTC/dump_xtc.cpp b/src/XTC/dump_xtc.cpp
index 0aed69ea7b..32d6f3cf3c 100644
--- a/src/XTC/dump_xtc.cpp
+++ b/src/XTC/dump_xtc.cpp
@@ -67,9 +67,11 @@ DumpXTC::DumpXTC(LAMMPS *lmp, int narg, char **arg) : Dump(lmp, narg, arg)
 
   // allocate global array for atom coords
 
-  if (igroup == 0) natoms = static_cast<int> (atom->natoms);
-  else natoms = static_cast<int> (group->count(igroup));
-  if (natoms <= 0) error->all("Invalid natoms for dump xtc");
+  bigint n;
+  if (igroup == 0) n = static_cast<int> (atom->natoms);
+  else n = static_cast<int> (group->count(igroup));
+  if (n > MAXSMALLINT) error->all("Too many atoms for dump xtc");
+  natoms = n;
 
   coords = (float *) memory->smalloc(3*natoms*sizeof(float),"dump:coords");
 
diff --git a/src/compute_reduce.cpp b/src/compute_reduce.cpp
index db9c3b6a6b..da1ff6bcbd 100644
--- a/src/compute_reduce.cpp
+++ b/src/compute_reduce.cpp
@@ -604,7 +604,7 @@ bigint ComputeReduce::count(int m)
     } else if (flavor[m] == LOCAL) {
       bigint ncount = compute->size_local_rows;
       bigint ncountall;
-      MPI_Allreduce(&ncount,&ncountall,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+      MPI_Allreduce(&ncount,&ncountall,1,MPI_LMP_BIGINT,MPI_SUM,world);
       return ncountall;
     }
   } else if (which[m] == FIX) {
@@ -614,7 +614,7 @@ bigint ComputeReduce::count(int m)
     } else if (flavor[m] == LOCAL) {
       bigint ncount = fix->size_local_rows;
       bigint ncountall;
-      MPI_Allreduce(&ncount,&ncountall,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+      MPI_Allreduce(&ncount,&ncountall,1,MPI_LMP_BIGINT,MPI_SUM,world);
       return ncountall;
     }
   } else if (which[m] == VARIABLE)
diff --git a/src/create_atoms.cpp b/src/create_atoms.cpp
index e895c68531..98acf8dda9 100644
--- a/src/create_atoms.cpp
+++ b/src/create_atoms.cpp
@@ -169,7 +169,7 @@ void CreateAtoms::command(int narg, char **arg)
   // new total # of atoms
 
   bigint nblocal = atom->nlocal;
-  MPI_Allreduce(&nblocal,&atom->natoms,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&nblocal,&atom->natoms,1,MPI_LMP_BIGINT,MPI_SUM,world);
 
   // print status
 
@@ -185,8 +185,8 @@ void CreateAtoms::command(int narg, char **arg)
   // if global map exists, reset it
   // if a molecular system, set nspecial to 0 for new atoms
 
-  if (atom->natoms > MAXINT32) atom->tag_enable = 0;
-  if (atom->natoms <= MAXINT32) atom->tag_extend();
+  if (atom->natoms > MAXTAGINT) atom->tag_enable = 0;
+  if (atom->natoms <= MAXTAGINT) atom->tag_extend();
 
   if (atom->map_style) {
     atom->nghost = 0;
diff --git a/src/delete_atoms.cpp b/src/delete_atoms.cpp
index 1fcf224b0f..d3a2bed8ea 100644
--- a/src/delete_atoms.cpp
+++ b/src/delete_atoms.cpp
@@ -93,7 +93,7 @@ void DeleteAtoms::command(int narg, char **arg)
   // set nghost to 0 so old ghosts of deleted atoms won't be mapped
 
   bigint nblocal = atom->nlocal;
-  MPI_Allreduce(&nblocal,&atom->natoms,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&nblocal,&atom->natoms,1,MPI_LMP_BIGINT,MPI_SUM,world);
   if (atom->map_style) {
     atom->nghost = 0;
     atom->map_init();
diff --git a/src/delete_bonds.cpp b/src/delete_bonds.cpp
index b2c80cd8f1..9bf92c512d 100644
--- a/src/delete_bonds.cpp
+++ b/src/delete_bonds.cpp
@@ -341,7 +341,7 @@ void DeleteBonds::command(int narg, char **arg)
     if (atom->avec->bonds_allow) {
       bigint nbonds = 0;
       for (i = 0; i < nlocal; i++) nbonds += atom->num_bond[i];
-      MPI_Allreduce(&nbonds,&atom->nbonds,1,MPI_UNSIGNED_LONG_LONG,
+      MPI_Allreduce(&nbonds,&atom->nbonds,1,MPI_LMP_BIGINT,
 		    MPI_SUM,world);
       if (force->newton_bond == 0) atom->nbonds /= 2;
     }
@@ -349,7 +349,7 @@ void DeleteBonds::command(int narg, char **arg)
     if (atom->avec->angles_allow) {
       bigint nangles = 0;
       for (i = 0; i < nlocal; i++) nangles += atom->num_angle[i];
-      MPI_Allreduce(&nangles,&atom->nangles,1,MPI_UNSIGNED_LONG_LONG,
+      MPI_Allreduce(&nangles,&atom->nangles,1,MPI_LMP_BIGINT,
 		    MPI_SUM,world);
       if (force->newton_bond == 0) atom->nangles /= 3;
     }
@@ -358,7 +358,7 @@ void DeleteBonds::command(int narg, char **arg)
       bigint ndihedrals = 0;
       for (i = 0; i < nlocal; i++) ndihedrals += atom->num_dihedral[i];
       MPI_Allreduce(&ndihedrals,&atom->ndihedrals,
-		    1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+		    1,MPI_LMP_BIGINT,MPI_SUM,world);
       if (force->newton_bond == 0) atom->ndihedrals /= 4;
     }
 
@@ -366,7 +366,7 @@ void DeleteBonds::command(int narg, char **arg)
       bigint nimpropers = 0;
       for (i = 0; i < nlocal; i++) nimpropers += atom->num_improper[i];
       MPI_Allreduce(&nimpropers,&atom->nimpropers,
-		    1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+		    1,MPI_LMP_BIGINT,MPI_SUM,world);
       if (force->newton_bond == 0) atom->nimpropers /= 4;
     }
 
@@ -386,9 +386,9 @@ void DeleteBonds::command(int narg, char **arg)
       for (m = 0; m < atom->num_bond[i]; m++)
 	if (atom->bond_type[i][m] > 0) bond_on++;
 	else bond_off++;
-    MPI_Allreduce(&bond_on,&tmp,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+    MPI_Allreduce(&bond_on,&tmp,1,MPI_LMP_BIGINT,MPI_SUM,world);
     bond_on = tmp;
-    MPI_Allreduce(&bond_off,&tmp,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+    MPI_Allreduce(&bond_off,&tmp,1,MPI_LMP_BIGINT,MPI_SUM,world);
     bond_off = tmp;
     if (force->newton_bond == 0) {
       bond_on /= 2;
@@ -402,9 +402,9 @@ void DeleteBonds::command(int narg, char **arg)
       for (m = 0; m < atom->num_angle[i]; m++)
 	if (atom->angle_type[i][m] > 0) angle_on++;
 	else angle_off++;
-    MPI_Allreduce(&angle_on,&tmp,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+    MPI_Allreduce(&angle_on,&tmp,1,MPI_LMP_BIGINT,MPI_SUM,world);
     angle_on = tmp;
-    MPI_Allreduce(&angle_off,&tmp,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+    MPI_Allreduce(&angle_off,&tmp,1,MPI_LMP_BIGINT,MPI_SUM,world);
     angle_off = tmp;
     if (force->newton_bond == 0) {
       angle_on /= 3;
@@ -418,9 +418,9 @@ void DeleteBonds::command(int narg, char **arg)
       for (m = 0; m < atom->num_dihedral[i]; m++)
 	if (atom->dihedral_type[i][m] > 0) dihedral_on++;
 	else dihedral_off++;
-    MPI_Allreduce(&dihedral_on,&tmp,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+    MPI_Allreduce(&dihedral_on,&tmp,1,MPI_LMP_BIGINT,MPI_SUM,world);
     dihedral_on = tmp;
-    MPI_Allreduce(&dihedral_off,&tmp,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+    MPI_Allreduce(&dihedral_off,&tmp,1,MPI_LMP_BIGINT,MPI_SUM,world);
     dihedral_off = tmp;
     if (force->newton_bond == 0) {
       dihedral_on /= 4;
@@ -434,9 +434,9 @@ void DeleteBonds::command(int narg, char **arg)
       for (m = 0; m < atom->num_improper[i]; m++)
 	if (atom->improper_type[i][m] > 0) improper_on++;
 	else improper_off++;
-    MPI_Allreduce(&improper_on,&tmp,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+    MPI_Allreduce(&improper_on,&tmp,1,MPI_LMP_BIGINT,MPI_SUM,world);
     improper_on = tmp;
-    MPI_Allreduce(&improper_off,&tmp,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+    MPI_Allreduce(&improper_off,&tmp,1,MPI_LMP_BIGINT,MPI_SUM,world);
     improper_off = tmp;
     if (force->newton_bond == 0) {
       improper_on /= 4;
diff --git a/src/displace_atoms.cpp b/src/displace_atoms.cpp
index 4a9be6df28..ff55334237 100644
--- a/src/displace_atoms.cpp
+++ b/src/displace_atoms.cpp
@@ -213,7 +213,7 @@ void DisplaceAtoms::command(int narg, char **arg)
 
   bigint natoms;
   bigint nblocal = atom->nlocal;
-  MPI_Allreduce(&nblocal,&natoms,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&nblocal,&natoms,1,MPI_LMP_BIGINT,MPI_SUM,world);
   if (natoms != atom->natoms) {
     char str[128];
     sprintf(str,"Lost atoms via displace_atoms: original %lu current %lu",
diff --git a/src/displace_box.cpp b/src/displace_box.cpp
index 33d1a3137c..ece73177d5 100644
--- a/src/displace_box.cpp
+++ b/src/displace_box.cpp
@@ -379,7 +379,7 @@ void DisplaceBox::command(int narg, char **arg)
 
   bigint natoms;
   bigint nblocal = atom->nlocal;
-  MPI_Allreduce(&nblocal,&natoms,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&nblocal,&natoms,1,MPI_LMP_BIGINT,MPI_SUM,world);
   if (natoms != atom->natoms) {
     char str[128];
     sprintf(str,"Lost atoms via displace_box: original %lu current %lu",
diff --git a/src/dump.cpp b/src/dump.cpp
index 76708f01eb..c0939f2453 100644
--- a/src/dump.cpp
+++ b/src/dump.cpp
@@ -174,7 +174,7 @@ void Dump::init()
       irregular = new Irregular(lmp);
 
     bigint size = group->count(igroup);
-    if (size > MAXINT32) error->all("Too many atoms to dump sort");
+    if (size > MAXSMALLINT) error->all("Too many atoms to dump sort");
 
     // set reorderflag = 1 if can simply reorder local atoms rather than sort
     // criteria: sorting by ID, atom IDs are consecutive from 1 to Natoms
diff --git a/src/dump_dcd.cpp b/src/dump_dcd.cpp
index 222b8dc4be..1a8743ae76 100644
--- a/src/dump_dcd.cpp
+++ b/src/dump_dcd.cpp
@@ -71,7 +71,7 @@ DumpDCD::DumpDCD(LAMMPS *lmp, int narg, char **arg) : Dump(lmp, narg, arg)
   bigint n;
   if (igroup == 0) n = static_cast<int> (atom->natoms);
   else n = static_cast<int> (group->count(igroup));
-  if (n > MAXINT32) error->all("Too many atoms for dump dcd");
+  if (n > MAXSMALLINT) error->all("Too many atoms for dump dcd");
   natoms = n;
 
   coords = (float *) memory->smalloc(3*natoms*sizeof(float),"dump:coords");
diff --git a/src/finish.cpp b/src/finish.cpp
index d45cf63b29..5d148a7be5 100644
--- a/src/finish.cpp
+++ b/src/finish.cpp
@@ -86,7 +86,7 @@ void Finish::end(int flag)
     // use actual natoms, in case atoms were lost
 
     bigint nblocal = atom->nlocal;
-    MPI_Allreduce(&nblocal,&natoms,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+    MPI_Allreduce(&nblocal,&natoms,1,MPI_LMP_BIGINT,MPI_SUM,world);
     
     if (me == 0) {
       if (screen) 
diff --git a/src/group.cpp b/src/group.cpp
index b2ee3c4afd..79a526b5b7 100644
--- a/src/group.cpp
+++ b/src/group.cpp
@@ -497,7 +497,7 @@ bigint Group::count(int igroup)
 
   bigint nsingle = n;
   bigint nall;
-  MPI_Allreduce(&nsingle,&nall,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&nsingle,&nall,1,MPI_LMP_BIGINT,MPI_SUM,world);
   return nall;
 }
 
@@ -520,7 +520,7 @@ bigint Group::count(int igroup, int iregion)
 
   bigint nsingle = n;
   bigint nall;
-  MPI_Allreduce(&nsingle,&nall,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&nsingle,&nall,1,MPI_LMP_BIGINT,MPI_SUM,world);
   return nall;
 }
 
diff --git a/src/lammps.cpp b/src/lammps.cpp
index 6b87c0a33b..920f336768 100644
--- a/src/lammps.cpp
+++ b/src/lammps.cpp
@@ -247,9 +247,9 @@ LAMMPS::LAMMPS(int narg, char **arg, MPI_Comm communicator)
     error->all("No support for 8-byte unsigned integers");
 
   int mpisize;
-  MPI_Type_size(MPI_UNSIGNED_LONG_LONG,&mpisize);
+  MPI_Type_size(MPI_LMP_BIGINT,&mpisize);
   if (mpisize != 8)
-    error->all("MPI_UNSIGNED_LONG_LONG is not 8-byte data type");
+    error->all("MPI_LMP_BIGINT is not 8-byte data type");
 
   // allocate input class now that MPI is fully setup
 
diff --git a/src/library.cpp b/src/library.cpp
index 546ad4f3f7..2b2cc39778 100644
--- a/src/library.cpp
+++ b/src/library.cpp
@@ -348,7 +348,7 @@ void *lammps_extract_variable(void *ptr, char *name, char *group)
 int lammps_get_natoms(void *ptr)
 {
   LAMMPS *lmp = (LAMMPS *) ptr;
-  if (lmp->atom->natoms > MAXINT32) return 0;
+  if (lmp->atom->natoms > MAXSMALLINT) return 0;
   int natoms = static_cast<int> (lmp->atom->natoms);
   return natoms;
 }
@@ -362,7 +362,7 @@ void lammps_get_coords(void *ptr, double *coords)
   // error if tags are not defined or not consecutive
 
   if (lmp->atom->tag_enable == 0 || lmp->atom->tag_consecutive() == 0) return;
-  if (lmp->atom->natoms > MAXINT32) return;
+  if (lmp->atom->natoms > MAXSMALLINT) return;
 
   int natoms = static_cast<int> (lmp->atom->natoms);
   double *copy = new double[3*natoms];
@@ -394,7 +394,7 @@ void lammps_put_coords(void *ptr, double *coords)
   // error if no map defined by LAMMPS
 
   if (lmp->atom->map_style == 0) return;
-  if (lmp->atom->natoms > MAXINT32) return;
+  if (lmp->atom->natoms > MAXSMALLINT) return;
 
   int natoms = static_cast<int> (lmp->atom->natoms);
   double **x = lmp->atom->x;
diff --git a/src/lmptype.h b/src/lmptype.h
index 1ab30c82cc..82c4644e7e 100644
--- a/src/lmptype.h
+++ b/src/lmptype.h
@@ -11,6 +11,23 @@
    See the README file in the top-level LAMMPS directory.
 ------------------------------------------------------------------------- */
 
+// define integer data types used by LAMMPS and associated size limits
+
+// smallint = variables for system on 1 processor (nlocal, etc)
+// tagint = variables for atom IDs (tag)
+// bigint = variables for total system and timesteps (natoms, ntimestep, etc)
+
+// smallint must be an int, as defined by C compiler
+// tagint can be 32-bit or 64-bit int, must be >= smallint
+// bigint can be 32-bit or 64-bit int, must be >= smallint and >= tagint
+
+// MAXSMALLINT = max value of a smallint
+// MAXTAGINT = max value of a tagint
+// MAXBIGINT = max value of a bigint
+
+// MPI_LMP_TAGINT = MPI data type corresponding to tagint
+// MPI_LMP_BIGINT = MPI data type corresponding to bigint
+
 #ifndef LMP_LMPTYPE_H
 #define LMP_LMPTYPE_H
 
@@ -18,8 +35,18 @@
 
 namespace LAMMPS_NS {
 
-typedef uint64_t bigint;
-#define MAXINT32 0x7FFFFFFF
+// default settings: 4-byte smallint, 4-byte tagint, 8-byte bigint
+
+typedef int smallint;
+typedef int tagint;
+typedef int64_t bigint;
+
+#define MAXSMALLINT 0x7FFFFFFF
+#define MAXTAGINT 0x7FFFFFFF
+#define MAXBIGINT 0x7FFFFFFFFFFFFFFFL
+
+#define MPI_LMP_TAGINT MPI_INT
+#define MPI_LMP_BIGINT MPI_LONG_LONG
 
 }
 
diff --git a/src/min.cpp b/src/min.cpp
index 8f58ceae55..a1e0a5bbff 100644
--- a/src/min.cpp
+++ b/src/min.cpp
@@ -202,7 +202,7 @@ void Min::setup()
   bigint ndofme = 3*atom->nlocal;
   for (int m = 0; m < nextra_atom; m++)
     ndofme += extra_peratom[m]*atom->nlocal;
-  MPI_Allreduce(&ndofme,&ndoftotal,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&ndofme,&ndoftotal,1,MPI_LMP_BIGINT,MPI_SUM,world);
   ndoftotal += nextra_global;
 
   // setup domain, communication and neighboring
diff --git a/src/min_cg.cpp b/src/min_cg.cpp
index 63ed35fddd..9ad87904fb 100644
--- a/src/min_cg.cpp
+++ b/src/min_cg.cpp
@@ -51,7 +51,7 @@ int MinCG::iterate(int maxiter)
   // nlimit = max # of CG iterations before restarting
   // set to ndoftotal unless too big
 
-  int nlimit = static_cast<int> (MIN(MAXINT32,ndoftotal));
+  int nlimit = static_cast<int> (MIN(MAXSMALLINT,ndoftotal));
 
   // initialize working vectors
 
diff --git a/src/read_data.cpp b/src/read_data.cpp
index f8229bdfd0..92a1fef909 100644
--- a/src/read_data.cpp
+++ b/src/read_data.cpp
@@ -435,7 +435,7 @@ void ReadData::atoms()
   // check that all atoms were assigned correctly
 
   bigint tmp = atom->nlocal;
-  MPI_Allreduce(&tmp,&natoms,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&tmp,&natoms,1,MPI_LMP_BIGINT,MPI_SUM,world);
 
   if (me == 0) {
     if (screen) fprintf(screen,"  %lu atoms\n",natoms);
@@ -568,7 +568,7 @@ void ReadData::bonds()
   bigint sum;
   bigint n = 0;
   for (i = 0; i < nlocal; i++) n += atom->num_bond[i];
-  MPI_Allreduce(&n,&sum,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&n,&sum,1,MPI_LMP_BIGINT,MPI_SUM,world);
   int factor = 1;
   if (!force->newton_bond) factor = 2;
 
@@ -613,7 +613,7 @@ void ReadData::angles()
   bigint sum;
   bigint n = 0;
   for (i = 0; i < nlocal; i++) n += atom->num_angle[i];
-  MPI_Allreduce(&n,&sum,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&n,&sum,1,MPI_LMP_BIGINT,MPI_SUM,world);
   int factor = 1;
   if (!force->newton_bond) factor = 3;
 
@@ -658,7 +658,7 @@ void ReadData::dihedrals()
   bigint sum;
   bigint n = 0;
   for (i = 0; i < nlocal; i++) n += atom->num_dihedral[i];
-  MPI_Allreduce(&n,&sum,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&n,&sum,1,MPI_LMP_BIGINT,MPI_SUM,world);
   int factor = 1;
   if (!force->newton_bond) factor = 4;
 
@@ -704,7 +704,7 @@ void ReadData::impropers()
   bigint sum;
   bigint n = 0;
   for (i = 0; i < nlocal; i++) n += atom->num_improper[i];
-  MPI_Allreduce(&n,&sum,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&n,&sum,1,MPI_LMP_BIGINT,MPI_SUM,world);
   int factor = 1;
   if (!force->newton_bond) factor = 4;
 
@@ -976,7 +976,7 @@ void ReadData::scan(int &bond_per_atom, int &angle_per_atom,
   int i,tmp1,tmp2,atom1,atom2,atom3,atom4;
   char *eof;
 
-  if (atom->natoms > MAXINT32)
+  if (atom->natoms > MAXSMALLINT)
     error->all("Molecular data file has too many atoms");
 
   int natoms = static_cast<int> (atom->natoms);
diff --git a/src/read_restart.cpp b/src/read_restart.cpp
index f325bc67b2..0078a8324d 100644
--- a/src/read_restart.cpp
+++ b/src/read_restart.cpp
@@ -293,7 +293,7 @@ void ReadRestart::command(int narg, char **arg)
 
   bigint natoms;
   bigint nblocal = atom->nlocal;
-  MPI_Allreduce(&nblocal,&natoms,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&nblocal,&natoms,1,MPI_LMP_BIGINT,MPI_SUM,world);
 
   if (me == 0) {
     if (screen) fprintf(screen,"  %lu atoms\n",natoms);
@@ -819,6 +819,6 @@ bigint ReadRestart::read_bigint()
 {
   bigint value;
   if (me == 0) fread(&value,sizeof(bigint),1,fp);
-  MPI_Bcast(&value,1,MPI_UNSIGNED_LONG_LONG,0,world);
+  MPI_Bcast(&value,1,MPI_LMP_BIGINT,0,world);
   return value;
 }
diff --git a/src/replicate.cpp b/src/replicate.cpp
index a4e02bd12a..7c8fc5e68e 100644
--- a/src/replicate.cpp
+++ b/src/replicate.cpp
@@ -129,16 +129,16 @@ void Replicate::command(int narg, char **arg)
   // if molecular, N/Nbonds/etc cannot be > 2^31 else tags/counts invalid
 
   double rep = nrep;
-  if (rep*old->natoms > MAXINT32) atom->tag_enable = 0;
+  if (rep*old->natoms > MAXSMALLINT) atom->tag_enable = 0;
 
   if (atom->tag_enable == 0)
     for (int i = 0; i < atom->nlocal; i++)
       atom->tag[i] = 0;
 
   if (atom->molecular) {
-    if (rep*old->natoms > MAXINT32 || rep*old->nbonds > MAXINT32 ||
-	rep*old->nangles > MAXINT32 || rep*old->ndihedrals > MAXINT32 ||
-	rep*old->nimpropers > MAXINT32)
+    if (rep*old->natoms > MAXBIGINT || rep*old->nbonds > MAXBIGINT ||
+	rep*old->nangles > MAXBIGINT || rep*old->ndihedrals > MAXBIGINT ||
+	rep*old->nimpropers > MAXBIGINT)
       error->all("Too big a problem to replicate with molecular atom style");
   }
 
@@ -365,7 +365,7 @@ void Replicate::command(int narg, char **arg)
 
   bigint natoms;
   bigint nblocal = atom->nlocal;
-  MPI_Allreduce(&nblocal,&natoms,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&nblocal,&natoms,1,MPI_LMP_BIGINT,MPI_SUM,world);
 
   if (me == 0) {
     if (screen) fprintf(screen,"  %lu atoms\n",natoms);
diff --git a/src/thermo.cpp b/src/thermo.cpp
index 042a28fcea..76af0c0d13 100644
--- a/src/thermo.cpp
+++ b/src/thermo.cpp
@@ -353,7 +353,7 @@ bigint Thermo::lost_check()
 
   bigint ntotal;
   bigint nblocal = atom->nlocal;
-  MPI_Allreduce(&nblocal,&ntotal,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&nblocal,&ntotal,1,MPI_LMP_BIGINT,MPI_SUM,world);
   if (ntotal == atom->natoms) return ntotal;
 
   // if not checking or already warned, just return
diff --git a/src/velocity.cpp b/src/velocity.cpp
index 0a9e944b08..4cadf22de6 100644
--- a/src/velocity.cpp
+++ b/src/velocity.cpp
@@ -223,7 +223,7 @@ void Velocity::create(double t_desired, int seed)
 
     // error check
 
-    if (atom->natoms > MAXINT32)
+    if (atom->natoms > MAXSMALLINT)
       error->all("Too big a problem to use velocity create loop all");
     if (atom->tag_enable == 0)
       error->all("Cannot use velocity create loop all unless atoms have IDs");
diff --git a/src/write_restart.cpp b/src/write_restart.cpp
index 25cba1c04d..3608ddfad6 100644
--- a/src/write_restart.cpp
+++ b/src/write_restart.cpp
@@ -122,7 +122,7 @@ void WriteRestart::write(char *file)
   // if unequal and thermo lostflag is "error", don't write restart file
 
   bigint nblocal = atom->nlocal;
-  MPI_Allreduce(&nblocal,&natoms,1,MPI_UNSIGNED_LONG_LONG,MPI_SUM,world);
+  MPI_Allreduce(&nblocal,&natoms,1,MPI_LMP_BIGINT,MPI_SUM,world);
   if (natoms != atom->natoms && output->thermo->lostflag == ERROR) 
     error->all("Atom count is inconsistent, cannot write restart file");