diff --git a/src/RIGID/fix_rigid_small.cpp b/src/RIGID/fix_rigid_small.cpp
index 4de368ca85..7c6aab4861 100644
--- a/src/RIGID/fix_rigid_small.cpp
+++ b/src/RIGID/fix_rigid_small.cpp
@@ -1576,8 +1576,9 @@ void FixRigidSmall::create_bodies(tagint *bodyID)
   // func = compute bbox of each body, find atom closest to geometric center
 
   char *buf;
-  int nreturn = comm->rendezvous(ncount,proclist,(char *) inbuf,sizeof(InRvous),
-                                 rendezvous_body,buf,sizeof(OutRvous),
+  int nreturn = comm->rendezvous(1,ncount,(char *) inbuf,sizeof(InRvous),
+                                 0,proclist,
+                                 rendezvous_body,0,buf,sizeof(OutRvous),
                                  (void *) this);
   OutRvous *outbuf = (OutRvous *) buf;
   
diff --git a/src/RIGID/fix_shake.cpp b/src/RIGID/fix_shake.cpp
index 51121f0853..35153de839 100644
--- a/src/RIGID/fix_shake.cpp
+++ b/src/RIGID/fix_shake.cpp
@@ -1068,9 +1068,9 @@ void FixShake::atom_owners()
   // each proc assigned every 1/Pth atom
   
   char *buf;
-  comm->rendezvous(nlocal,proclist, 
-                   (char *) idbuf,sizeof(IDRvous),
-                   rendezvous_ids,buf,0,(void *) this);
+  comm->rendezvous(1,nlocal,(char *) idbuf,sizeof(IDRvous),
+                   0,proclist,
+                   rendezvous_ids,0,buf,0,(void *) this);
 
   memory->destroy(proclist);
   memory->sfree(idbuf);
@@ -1174,9 +1174,10 @@ void FixShake::partner_info(int *npartner, tagint **partner_tag,
   // receives all data needed to populate un-owned partner 4 values
 
   char *buf;
-  int nreturn = comm->rendezvous(nsend,proclist, 
-                                 (char *) inbuf,sizeof(PartnerInfo),
-                                 rendezvous_partners_info,buf,sizeof(PartnerInfo),
+  int nreturn = comm->rendezvous(1,nsend,(char *) inbuf,sizeof(PartnerInfo),
+                                 0,proclist,
+                                 rendezvous_partners_info,
+                                 0,buf,sizeof(PartnerInfo),
                                  (void *) this);
   PartnerInfo *outbuf = (PartnerInfo *) buf;
     
@@ -1194,7 +1195,8 @@ void FixShake::partner_info(int *npartner, tagint **partner_tag,
     partner_type[i][j] = outbuf[m].type;
     partner_massflag[i][j] = outbuf[m].massflag;
 
-    // only set partner_bondtype if my atom did not set it when setting up rendezvous
+    // only set partner_bondtype if my atom did not set it
+    //   when setting up rendezvous
     // if this proc set it, then sender of this datum set outbuf.bondtype = 0
     
     if (partner_bondtype[i][j] == 0)
@@ -1261,9 +1263,9 @@ void FixShake::nshake_info(int *npartner, tagint **partner_tag,
   // receives all data needed to populate un-owned partner nshake
 
   char *buf;
-  int nreturn = comm->rendezvous(nsend,proclist, 
-                                 (char *) inbuf,sizeof(NShakeInfo),
-                                 rendezvous_nshake,buf,sizeof(NShakeInfo),
+  int nreturn = comm->rendezvous(1,nsend,(char *) inbuf,sizeof(NShakeInfo),
+                                 0,proclist,
+                                 rendezvous_nshake,0,buf,sizeof(NShakeInfo),
                                  (void *) this);
   NShakeInfo *outbuf = (NShakeInfo *) buf;
     
@@ -1354,9 +1356,9 @@ void FixShake::shake_info(int *npartner, tagint **partner_tag,
   // receives all data needed to populate un-owned shake info
 
   char *buf;
-  int nreturn = comm->rendezvous(nsend,proclist, 
-                                 (char *) inbuf,sizeof(ShakeInfo),
-                                 rendezvous_shake,buf,sizeof(ShakeInfo),
+  int nreturn = comm->rendezvous(1,nsend,(char *) inbuf,sizeof(ShakeInfo),
+                                 0,proclist,
+                                 rendezvous_shake,0,buf,sizeof(ShakeInfo),
                                  (void *) this);
   ShakeInfo *outbuf = (ShakeInfo *) buf;
     
@@ -1412,7 +1414,7 @@ int FixShake::rendezvous_ids(int n, char *inbuf,
   fsptr->atomIDs = atomIDs;
   fsptr->procowner = procowner;
 
-  // flag = 0: no 2nd irregular comm needed in comm->rendezvous
+  // flag = 0: no second comm needed in rendezvous
   
   flag = 0;
   return 0;
diff --git a/src/comm.cpp b/src/comm.cpp
index 9bdaf0798a..39d4311aa2 100644
--- a/src/comm.cpp
+++ b/src/comm.cpp
@@ -729,34 +729,78 @@ void Comm::ring(int n, int nper, void *inbuf, int messtag,
 /* ----------------------------------------------------------------------
    rendezvous communication operation
    three stages:
-     first Irregular converts inbuf from caller decomp to rvous decomp
+     first comm sends inbuf from caller decomp to rvous decomp
      callback operates on data in rendevous decomp
-     last Irregular converts outbuf from rvous decomp back to caller decomp
+     second comm sends outbuf from rvous decomp back to caller decomp
    inputs:
-     n = # of input datums
-     proclist = proc that owns each input datum in rendezvous decomposition
-     inbuf = list of input datums
-     insize = size in bytes of each input datum
+     which = perform (0) irregular or (1) MPI_All2allv communication
+     n = # of datums in inbuf
+     inbuf = vector of input datums
+     insize = byte size of each input datum
+     inorder = 0 for inbuf in random proc order, 1 for datums ordered by proc
+     procs: inorder 0 = proc to send each datum to, 1 = # of datums/proc, 
      callback = caller function to invoke in rendezvous decomposition
+                takes input datums, returns output datums
+     outorder = same as inorder, but for datums returned by callback()
+     ptr = pointer to caller class, passed to callback()
    outputs:
      nout = # of output datums (function return)
-     outbuf = list of output datums
-     outsize = size in bytes of each output datum
+     outbuf = vector of output datums
+     outsize = byte size of each output datum
+   callback inputs:
+     nrvous = # of rvous decomp datums in inbuf_rvous
+     inbuf_rvous = vector of rvous decomp input datums
+     ptr = pointer to caller class
+   callback outputs:
+     nrvous_out = # of rvous decomp output datums (function return)
+     flag = 0 for no second comm, 1 for outbuf_rvous = inbuf_rvous,
+            2 for second comm with new outbuf_rvous
+     procs_rvous = outorder 0 = proc to send each datum to, 1 = # of datums/proc
+                   allocated
+     outbuf_rvous = vector of rvous decomp output datums
+   NOTE: could use MPI_INT or MPI_DOUBLE insead of MPI_CHAR
+         to avoid checked-for overflow in MPI_Alltoallv?
 ------------------------------------------------------------------------- */
 
-int Comm::rendezvous(int n, int *proclist, char *inbuf, int insize,
-                     int (*callback)(int, char *, int &, int *&, char *&, void *),
-                     char *&outbuf, int outsize, void *ptr)
+int Comm::
+rendezvous(int which, int n, char *inbuf, int insize,
+           int inorder, int *procs,
+           int (*callback)(int, char *, int &, int *&, char *&, void *),
+           int outorder, char *&outbuf, int outsize, void *ptr)
 {
-  // comm inbuf from caller decomposition to rendezvous decomposition
+  int nout;
 
+  if (which == 0)
+    nout = rendezvous_irregular(n,inbuf,insize,inorder,procs,callback,
+                                outorder,outbuf,outsize,ptr);
+  else
+    nout = rendezvous_all2all(n,inbuf,insize,inorder,procs,callback,
+                              outorder,outbuf,outsize,ptr);
+
+  return nout;
+}
+
+/* ---------------------------------------------------------------------- */
+
+int Comm::
+rendezvous_irregular(int n, char *inbuf, int insize, int inorder, int *procs,
+                     int (*callback)(int, char *, int &, int *&, char *&, void *),
+                     int outorder, char *&outbuf, 
+                     int outsize, void *ptr)
+{
+  // irregular comm of inbuf from caller decomp to rendezvous decomp
+  
   Irregular *irregular = new Irregular(lmp);
 
-  int n_rvous = irregular->create_data(n,proclist);  // add sort
-  char *inbuf_rvous = (char *) memory->smalloc((bigint) n_rvous*insize,
-                                               "rendezvous:inbuf_rvous");
+  int nrvous;
+  if (inorder) nrvous = irregular->create_data_grouped(n,procs);
+  else nrvous = irregular->create_data(n,procs);
+
+  char *inbuf_rvous = (char *) memory->smalloc((bigint) nrvous*insize,
+                                               "rendezvous:inbuf");
   irregular->exchange_data(inbuf,insize,inbuf_rvous);
 
+  bigint irregular1_bytes = 0; //irregular->irregular_bytes;
   irregular->destroy_data();
   delete irregular;
 
@@ -764,29 +808,253 @@ int Comm::rendezvous(int n, int *proclist, char *inbuf, int insize,
   // callback() allocates/populates proclist_rvous and outbuf_rvous
 
   int flag;
-  int *proclist_rvous;
+  int *procs_rvous;
   char *outbuf_rvous;
-
-  int nout_rvous = 
-    callback(n_rvous,inbuf_rvous,flag,proclist_rvous,outbuf_rvous,ptr);
+  int nrvous_out = callback(nrvous,inbuf_rvous,flag,
+                            procs_rvous,outbuf_rvous,ptr);
 
   if (flag != 1) memory->sfree(inbuf_rvous);  // outbuf_rvous = inbuf_vous
-  if (flag == 0) return 0;    // all nout_rvous are 0, no 2nd irregular
+  if (flag == 0) return 0;    // all nout_rvous are 0, no 2nd comm stage
 
-  // comm outbuf from rendezvous decomposition back to caller
+  // irregular comm of outbuf from rendezvous decomp back to caller decomp
   // caller will free outbuf
 
   irregular = new Irregular(lmp);
   
-  int nout = irregular->create_data(nout_rvous,proclist_rvous);
-  outbuf = (char *) memory->smalloc((bigint) nout*outsize,"rendezvous:outbuf");
+  int nout;
+  if (outorder) 
+    nout = irregular->create_data_grouped(nrvous_out,procs_rvous);
+  else nout = irregular->create_data(nrvous_out,procs_rvous);
+
+  outbuf = (char *) memory->smalloc((bigint) nout*outsize,
+                                    "rendezvous:outbuf");
   irregular->exchange_data(outbuf_rvous,outsize,outbuf);
-  
+
+  bigint irregular2_bytes = 0; //irregular->irregular_bytes;
   irregular->destroy_data();
   delete irregular;
-  memory->destroy(proclist_rvous);
+
+  memory->destroy(procs_rvous);
   memory->sfree(outbuf_rvous);
 
+  // approximate memory tally
+
+  bigint rvous_bytes = 0;
+  rvous_bytes += n*insize;                                // inbuf
+  rvous_bytes += nout*outsize;                            // outbuf
+  rvous_bytes += nrvous*insize;                           // inbuf_rvous
+  rvous_bytes += nrvous_out*outsize;                      // outbuf_rvous
+  rvous_bytes += nrvous_out*sizeof(int);                  // procs_rvous
+  rvous_bytes += MAX(irregular1_bytes,irregular2_bytes);  // max of 2 comms
+
+  // return number of output datums
+
+  return nout;
+}
+
+/* ---------------------------------------------------------------------- */
+
+int Comm::
+rendezvous_all2all(int n, char *inbuf, int insize, int inorder, int *procs,
+                   int (*callback)(int, char *, int &, int *&, char *&, void *),
+                   int outorder, char *&outbuf, int outsize, void *ptr)
+{
+  int iproc;
+  bigint all2all1_bytes,all2all2_bytes;
+  int *sendcount,*sdispls,*recvcount,*rdispls;
+  int *procs_a2a;
+  bigint *offsets;
+  char *inbuf_a2a,*outbuf_a2a;
+
+  // create procs and inbuf for All2all if necesary
+  
+  if (!inorder) {
+    memory->create(procs_a2a,nprocs,"rendezvous:procs");
+    inbuf_a2a = (char *) memory->smalloc((bigint) n*insize,
+                                         "rendezvous:inbuf");
+    memory->create(offsets,nprocs,"rendezvous:offsets");
+
+    for (int i = 0; i < nprocs; i++) procs_a2a[i] = 0;
+    for (int i = 0; i < n; i++) procs_a2a[procs[i]]++;
+
+    offsets[0] = 0;
+    for (int i = 1; i < nprocs; i++)
+      offsets[i] = offsets[i-1] + insize*procs_a2a[i-1];
+
+    bigint offset = 0;
+    for (int i = 0; i < n; i++) {
+      iproc = procs[i];
+      memcpy(&inbuf_a2a[offsets[iproc]],&inbuf[offset],insize);
+      offsets[iproc] += insize;
+      offset += insize;
+    }
+
+    all2all1_bytes = nprocs*sizeof(int) + nprocs*sizeof(bigint) + n*insize;
+
+  } else {
+    procs_a2a = procs;
+    inbuf_a2a = inbuf;
+    all2all1_bytes = 0;
+  }
+
+  // create args for MPI_Alltoallv() on input data
+  
+  memory->create(sendcount,nprocs,"rendezvous:sendcount");
+  memcpy(sendcount,procs_a2a,nprocs*sizeof(int));
+  
+  memory->create(recvcount,nprocs,"rendezvous:recvcount");
+  MPI_Alltoall(sendcount,1,MPI_INT,recvcount,1,MPI_INT,world);
+
+  memory->create(sdispls,nprocs,"rendezvous:sdispls");
+  memory->create(rdispls,nprocs,"rendezvous:rdispls");
+  sdispls[0] = rdispls[0] = 0;
+  for (int i = 1; i < nprocs; i++) {
+    sdispls[i] = sdispls[i-1] + sendcount[i-1];
+    rdispls[i] = rdispls[i-1] + recvcount[i-1];
+  }
+  int nrvous = rdispls[nprocs-1] + recvcount[nprocs-1];
+
+  // test for overflow of input data due to imbalance or insize
+  // means that individual sdispls or rdispls values overflow
+  
+  int overflow = 0;
+  if ((bigint) n*insize > MAXSMALLINT) overflow = 1;
+  if ((bigint) nrvous*insize > MAXSMALLINT) overflow = 1;
+  int overflowall;
+  MPI_Allreduce(&overflow,&overflowall,1,MPI_INT,MPI_MAX,world);
+  if (overflowall) error->all(FLERR,"Overflow input size in rendezvous_a2a");
+  
+  for (int i = 0; i < nprocs; i++) {
+    sendcount[i] *= insize;
+    sdispls[i] *= insize;
+    recvcount[i] *= insize;
+    rdispls[i] *= insize;
+  }
+
+  // all2all comm of inbuf from caller decomp to rendezvous decomp
+
+  char *inbuf_rvous = (char *) memory->smalloc((bigint) nrvous*insize,
+                                               "rendezvous:inbuf");
+
+  MPI_Alltoallv(inbuf_a2a,sendcount,sdispls,MPI_CHAR,
+		inbuf_rvous,recvcount,rdispls,MPI_CHAR,world);
+
+  if (!inorder) {
+    memory->destroy(procs_a2a);
+    memory->sfree(inbuf_a2a);
+    memory->destroy(offsets);
+  }
+
+  // peform rendezvous computation via callback()
+  // callback() allocates/populates proclist_rvous and outbuf_rvous
+
+  int flag;
+  int *procs_rvous;
+  char *outbuf_rvous;
+
+  int nrvous_out = callback(nrvous,inbuf_rvous,flag,
+                            procs_rvous,outbuf_rvous,ptr);
+
+  if (flag != 1) memory->sfree(inbuf_rvous);  // outbuf_rvous = inbuf_vous
+  if (flag == 0) return 0;    // all nout_rvous are 0, no 2nd irregular
+
+  // create procs and outbuf for All2all if necesary
+  
+  if (!outorder) {
+    memory->create(procs_a2a,nprocs,"rendezvous_a2a:procs");
+
+    outbuf_a2a = (char *) memory->smalloc((bigint) nrvous_out*outsize,
+                                          "rendezvous:outbuf");
+    memory->create(offsets,nprocs,"rendezvous:offsets");
+
+    for (int i = 0; i < nprocs; i++) procs_a2a[i] = 0;
+    for (int i = 0; i < nrvous_out; i++) procs_a2a[procs_rvous[i]]++;
+
+    offsets[0] = 0;
+    for (int i = 1; i < nprocs; i++)
+      offsets[i] = offsets[i-1] + outsize*procs_a2a[i-1];
+
+    bigint offset = 0;
+    for (int i = 0; i < nrvous_out; i++) {
+      iproc = procs_rvous[i];
+      memcpy(&outbuf_a2a[offsets[iproc]],&outbuf_rvous[offset],outsize);
+      offsets[iproc] += outsize;
+      offset += outsize;
+    }
+    
+    all2all2_bytes = nprocs*sizeof(int) + nprocs*sizeof(bigint) + 
+      nrvous_out*outsize;
+
+  } else {
+    procs_a2a = procs_rvous;
+    outbuf_a2a = outbuf_rvous;
+    all2all2_bytes = 0;
+  }
+
+  // comm outbuf from rendezvous decomposition back to caller
+
+  memcpy(sendcount,procs_a2a,nprocs*sizeof(int));
+  
+  MPI_Alltoall(sendcount,1,MPI_INT,recvcount,1,MPI_INT,world);
+  
+  sdispls[0] = rdispls[0] = 0;
+  for (int i = 1; i < nprocs; i++) {
+    sdispls[i] = sdispls[i-1] + sendcount[i-1];
+    rdispls[i] = rdispls[i-1] + recvcount[i-1];
+  }
+  int nout = rdispls[nprocs-1] + recvcount[nprocs-1];
+
+  // test for overflow of outbuf due to imbalance or outsize
+  // means that individual sdispls or rdispls values overflow
+  
+  overflow = 0;
+  if ((bigint) nrvous*outsize > MAXSMALLINT) overflow = 1;
+  if ((bigint) nout*outsize > MAXSMALLINT) overflow = 1;
+  MPI_Allreduce(&overflow,&overflowall,1,MPI_INT,MPI_MAX,world);
+  if (overflowall) error->all(FLERR,"Overflow output in rendezvous_a2a");
+  
+  for (int i = 0; i < nprocs; i++) {
+    sendcount[i] *= outsize;
+    sdispls[i] *= outsize;
+    recvcount[i] *= outsize;
+    rdispls[i] *= outsize;
+  }
+
+  // all2all comm of outbuf from rendezvous decomp back to caller decomp
+  // caller will free outbuf
+
+  outbuf = (char *) memory->smalloc((bigint) nout*outsize,"rendezvous:outbuf");
+
+  MPI_Alltoallv(outbuf_a2a,sendcount,sdispls,MPI_CHAR,
+		outbuf,recvcount,rdispls,MPI_CHAR,world);
+
+  memory->destroy(procs_rvous);
+  memory->sfree(outbuf_rvous);
+
+  if (!outorder) {
+    memory->destroy(procs_a2a);
+    memory->sfree(outbuf_a2a);
+    memory->destroy(offsets);
+  }
+
+  // clean up
+
+  memory->destroy(sendcount);
+  memory->destroy(recvcount);
+  memory->destroy(sdispls);
+  memory->destroy(rdispls);
+
+  // approximate memory tally
+
+  bigint rvous_bytes = 0;
+  rvous_bytes += n*insize;                                // inbuf
+  rvous_bytes += nout*outsize;                            // outbuf
+  rvous_bytes += nrvous*insize;                           // inbuf_rvous
+  rvous_bytes += nrvous_out*outsize;                      // outbuf_rvous
+  rvous_bytes += nrvous_out*sizeof(int);                  // procs_rvous
+  rvous_bytes += 4*nprocs*sizeof(int);                    // all2all vectors
+  rvous_bytes += MAX(all2all1_bytes,all2all2_bytes);      // reorder ops
+
   // return number of datums
 
   return nout;
diff --git a/src/comm.h b/src/comm.h
index a1bac53ac8..807da9bf0d 100644
--- a/src/comm.h
+++ b/src/comm.h
@@ -109,9 +109,9 @@ class Comm : protected Pointers {
 
   void ring(int, int, void *, int, void (*)(int, char *, void *),
             void *, void *, int self = 1);
-  int rendezvous(int, int *, char *, int, 
+  int rendezvous(int, int, char *, int, int, int *, 
                  int (*)(int, char *, int &, int *&, char *&, void *), 
-                 char *&, int, void *);
+                 int, char *&, int, void *);
 
   int read_lines_from_file(FILE *, int, int, char *);
   int read_lines_from_file_universe(FILE *, int, int, char *);
@@ -146,6 +146,14 @@ class Comm : protected Pointers {
   int ncores;                       // # of cores per node
   int coregrid[3];                  // 3d grid of cores within a node
   int user_coregrid[3];             // user request for cores in each dim
+
+  int rendezvous_irregular(int, char *, int, int, int *, 
+                           int (*)(int, char *, int &, int *&, char *&, void *), 
+                           int, char *&, int, void *);
+  int rendezvous_all2all(int, char *, int, int, int *, 
+                         int (*)(int, char *, int &, int *&, char *&, void *), 
+                         int, char *&, int, void *);
+
  public:
   enum{MULTIPLE};
 };
diff --git a/src/irregular.cpp b/src/irregular.cpp
index 60025249cf..77278ec4c5 100644
--- a/src/irregular.cpp
+++ b/src/irregular.cpp
@@ -622,6 +622,7 @@ int Irregular::create_data(int n, int *proclist, int sortflag)
   num_send = new int[nsend_proc];
   index_send = new int[n-work1[me]];
   index_self = new int[work1[me]];
+  maxindex = n;
 
   // proc_send = procs I send to
   // num_send = # of datums I send to each proc
@@ -679,8 +680,182 @@ int Irregular::create_data(int n, int *proclist, int sortflag)
 
   // receive incoming messages
   // proc_recv = procs I recv from
-  // num_recv = total size of message each proc sends me
-  // nrecvdatum = total size of data I recv
+  // num_recv = # of datums each proc sends me
+  // nrecvdatum = total # of datums I recv
+
+  int nrecvdatum = 0;
+  for (i = 0; i < nrecv_proc; i++) {
+    MPI_Recv(&num_recv[i],1,MPI_INT,MPI_ANY_SOURCE,0,world,status);
+    proc_recv[i] = status->MPI_SOURCE;
+    nrecvdatum += num_recv[i];
+  }
+  nrecvdatum += num_self;
+
+  // sort proc_recv and num_recv by proc ID if requested
+  // useful for debugging to insure reproducible ordering of received datums
+
+  if (sortflag) {
+    int *order = new int[nrecv_proc];
+    int *proc_recv_ordered = new int[nrecv_proc];
+    int *num_recv_ordered = new int[nrecv_proc];
+
+    for (i = 0; i < nrecv_proc; i++) order[i] = i;
+
+#if defined(LMP_QSORT)
+    proc_recv_copy = proc_recv;
+    qsort(order,nrecv_proc,sizeof(int),compare_standalone);
+#else
+    merge_sort(order,nrecv_proc,(void *)proc_recv,compare_standalone);
+#endif
+
+    int j;
+    for (i = 0; i < nrecv_proc; i++) {
+      j = order[i];
+      proc_recv_ordered[i] = proc_recv[j];
+      num_recv_ordered[i] = num_recv[j];
+    }
+
+    memcpy(proc_recv,proc_recv_ordered,nrecv_proc*sizeof(int));
+    memcpy(num_recv,num_recv_ordered,nrecv_proc*sizeof(int));
+    delete [] order;
+    delete [] proc_recv_ordered;
+    delete [] num_recv_ordered;
+  }
+
+  // barrier to insure all MPI_ANY_SOURCE messages are received
+  // else another proc could proceed to exchange_data() and send to me
+
+  MPI_Barrier(world);
+
+  // return # of datums I will receive
+
+  return nrecvdatum;
+}
+
+/* ----------------------------------------------------------------------
+   create communication plan based on list of datums of uniform size
+   n = # of datums to send
+   procs = how many datums to send to each proc, must include self
+   sort = flag for sorting order of received messages by proc ID
+   return total # of datums I will recv, including any to self
+------------------------------------------------------------------------- */
+
+int Irregular::create_data_grouped(int n, int *procs, int sortflag)
+{
+  int i,j,k,m;
+
+  // setup for collective comm
+  // work1 = # of datums I send to each proc, set self to 0
+  // work2 = 1 for all procs, used for ReduceScatter
+
+  for (i = 0; i < nprocs; i++) {
+    work1[i] = procs[i];
+    work2[i] = 1;
+  }
+  work1[me] = 0;
+
+  // nrecv_proc = # of procs I receive messages from, not including self
+  // options for performing ReduceScatter operation
+  // some are more efficient on some machines at big sizes
+
+#ifdef LAMMPS_RS_ALLREDUCE_INPLACE
+  MPI_Allreduce(MPI_IN_PLACE,work1,nprocs,MPI_INT,MPI_SUM,world);
+  nrecv_proc = work1[me];
+#else 
+#ifdef LAMMPS_RS_ALLREDUCE
+  MPI_Allreduce(work1,work2,nprocs,MPI_INT,MPI_SUM,world);
+  nrecv_proc = work2[me];
+#else
+  MPI_Reduce_scatter(work1,&nrecv_proc,work2,MPI_INT,MPI_SUM,world);
+#endif
+#endif
+
+  // allocate receive arrays
+
+  proc_recv = new int[nrecv_proc];
+  num_recv = new int[nrecv_proc];
+  request = new MPI_Request[nrecv_proc];
+  status = new MPI_Status[nrecv_proc];
+
+  // work1 = # of datums I send to each proc, including self
+  // nsend_proc = # of procs I send messages to, not including self
+
+  for (i = 0; i < nprocs; i++) work1[i] = procs[i];
+
+  nsend_proc = 0;
+  for (i = 0; i < nprocs; i++)
+    if (work1[i]) nsend_proc++;
+  if (work1[me]) nsend_proc--;
+
+  // allocate send and self arrays
+
+  proc_send = new int[nsend_proc];
+  num_send = new int[nsend_proc];
+  index_send = new int[n-work1[me]];
+  index_self = new int[work1[me]];
+  maxindex = n;
+
+  // proc_send = procs I send to
+  // num_send = # of datums I send to each proc
+  // num_self = # of datums I copy to self
+  // to balance pattern of send messages:
+  //   each proc begins with iproc > me, continues until iproc = me
+  // reset work1 to store which send message each proc corresponds to
+
+  int iproc = me;
+  int isend = 0;
+  for (i = 0; i < nprocs; i++) {
+    iproc++;
+    if (iproc == nprocs) iproc = 0;
+    if (iproc == me) {
+      num_self = work1[iproc];
+      work1[iproc] = 0;
+    } else if (work1[iproc] > 0) {
+      proc_send[isend] = iproc;
+      num_send[isend] = work1[iproc];
+      work1[iproc] = isend;
+      isend++;
+    }
+  }
+
+  // work2 = offsets into index_send for each proc I send to
+  // m = ptr into index_self
+  // index_send = list of which datums to send to each proc
+  //   1st N1 values are datum indices for 1st proc,
+  //   next N2 values are datum indices for 2nd proc, etc
+  // index_self = list of which datums to copy to self
+
+  work2[0] = 0;
+  for (i = 1; i < nsend_proc; i++) work2[i] = work2[i-1] + num_send[i-1];
+
+  m = 0;
+  i = 0;
+  for (iproc = 0; iproc < nprocs; iproc++) {
+    k = procs[iproc];
+    for (j = 0; j < k; j++) {
+      if (iproc == me) index_self[m++] = i++;
+      else {
+        isend = work1[iproc];
+        index_send[work2[isend]++] = i++;
+      }
+    }
+  }
+
+  // tell receivers how much data I send
+  // sendmax_proc = largest # of datums I send in a single message
+
+  sendmax_proc = 0;
+  for (i = 0; i < nsend_proc; i++) {
+    MPI_Request tmpReq; // Use non-blocking send to avoid possible deadlock
+    MPI_Isend(&num_send[i],1,MPI_INT,proc_send[i],0,world,&tmpReq);
+    MPI_Request_free(&tmpReq); // the MPI_Barrier below marks completion
+    sendmax_proc = MAX(sendmax_proc,num_send[i]);
+  }
+
+  // receive incoming messages
+  // proc_recv = procs I recv from
+  // num_recv = # of datums each proc sends me
+  // nrecvdatum = total # of datums I recv
 
   int nrecvdatum = 0;
   for (i = 0; i < nrecv_proc; i++) {
@@ -789,6 +964,12 @@ void Irregular::exchange_data(char *sendbuf, int nbytes, char *recvbuf)
   // wait on all incoming messages
 
   if (nrecv_proc) MPI_Waitall(nrecv_proc,request,status);
+
+  // approximate memory tally
+
+  bigint irregular_bytes = 2*nprocs*sizeof(int);
+  irregular_bytes += maxindex*sizeof(int);
+  irregular_bytes += maxbuf;
 }
 
 /* ----------------------------------------------------------------------
diff --git a/src/irregular.h b/src/irregular.h
index 1f74fe801b..d56bcb253d 100644
--- a/src/irregular.h
+++ b/src/irregular.h
@@ -33,6 +33,7 @@ class Irregular : protected Pointers {
                      int *procassign = NULL);
   int migrate_check();
   int create_data(int, int *, int sortflag = 0);
+  int create_data_grouped(int, int *, int sortflag = 0);
   void exchange_data(char *, int, char *);
   void destroy_data();
   bigint memory_usage();
@@ -48,6 +49,7 @@ class Irregular : protected Pointers {
   double *dbuf;                     // double buf for largest single atom send
   int maxbuf;                       // size of char buf in bytes
   char *buf;                        // char buf for largest single data send
+  int maxindex;                     // combined size of index_send + index_self
 
   int *mproclist,*msizes;           // persistent vectors in migrate_atoms
   int maxlocal;                     // allocated size of mproclist and msizes
diff --git a/src/special.cpp b/src/special.cpp
index b0d5bc7dca..34685d8c65 100644
--- a/src/special.cpp
+++ b/src/special.cpp
@@ -172,10 +172,9 @@ void Special::atom_owners()
   IDRvous *idbuf = (IDRvous *) 
     memory->smalloc((bigint) nlocal*sizeof(IDRvous),"special:idbuf");
 
-  // setup input buf to rendezvous comm
-  // input datums = pairs of bonded atoms
-  // owning proc for each datum = random hash of atomID
+  // setup input buf for rendezvous comm
   // one datum for each owned atom: datum = owning proc, atomID
+  // each proc assigned every 1/Pth atom
 
   for (int i = 0; i < nlocal; i++) {
     proclist[i] = tag[i] % nprocs;
@@ -184,19 +183,18 @@ void Special::atom_owners()
   }
 
   // perform rendezvous operation
-  // each proc assigned every 1/Pth atom
   
   char *buf;
-  comm->rendezvous(nlocal,proclist, 
-                   (char *) idbuf,sizeof(IDRvous),
-                   rendezvous_ids,buf,0,(void *) this);
+  comm->rendezvous(1,nlocal,(char *) idbuf,sizeof(IDRvous),0,proclist,
+                   rendezvous_ids,0,buf,0,(void *) this);
 
   memory->destroy(proclist);
   memory->sfree(idbuf);
 }
 
 /* ----------------------------------------------------------------------
-   onetwo build
+   onetwo build when newton_bond flag on
+   uses rendezvous comm
 ------------------------------------------------------------------------- */
 
 void Special::onetwo_build_newton()
@@ -225,9 +223,8 @@ void Special::onetwo_build_newton()
     memory->smalloc((bigint) nsend*sizeof(PairRvous),"special:inbuf");
 
   // setup input buf to rendezvous comm
-  // input datums = pairs of bonded atoms
-  // owning proc for each datum = atomID % nprocs
-  // one datum for each bond partner: bond partner ID, atomID
+  // one datum for each unowned bond partner: bond partner ID, atomID
+  // owning proc for each datum = bond partner ID % nprocs
 
   nsend = 0;
   for (i = 0; i < nlocal; i++) {
@@ -242,19 +239,19 @@ void Special::onetwo_build_newton()
   }
   
   // perform rendezvous operation
-  // each proc owns random subset of atoms
 
   char *buf;
-  int nreturn = comm->rendezvous(nsend,proclist, 
-                                 (char *) inbuf,sizeof(PairRvous),
-                                 rendezvous_pairs,buf,sizeof(PairRvous),
+  int nreturn = comm->rendezvous(1,nsend,(char *) inbuf,sizeof(PairRvous),
+                                 0,proclist,
+                                 rendezvous_pairs,0,buf,sizeof(PairRvous),
                                  (void *) this);
   PairRvous *outbuf = (PairRvous *) buf;
     
   memory->destroy(proclist);
   memory->sfree(inbuf);
 
-  // set nspecial[0] and onetwo for all owned atoms based on output info
+  // set nspecial[0] and onetwo for all owned atoms
+  // based on owned info plus rendezvous output info
   // output datums = pairs of atoms that are 1-2 neighbors
 
   for (i = 0; i < nlocal; i++) {
@@ -327,8 +324,8 @@ void Special::onetwo_build_newton_off()
 }
 
 /* ----------------------------------------------------------------------
-   onetwo build with newton_bond flag off
-   no need for rendezvous comm
+   onethree build
+   uses rendezvous comm
 ------------------------------------------------------------------------- */
 
 void Special::onethree_build()
@@ -355,10 +352,10 @@ void Special::onethree_build()
     memory->smalloc((bigint) nsend*sizeof(PairRvous),"special:inbuf");
 
   // setup input buf to rendezvous comm
-  // input datums = all pairs of onetwo atoms (they are 1-3 neighbors)
-  // owning proc for each datum = random hash of atomID
-  // one datum for each owned atom: datum = owning proc, atomID
-  // one datum for each onetwo pair: datum = atomID1, atomID2
+  // datums = pairs of onetwo partners where either is unknown
+  //          these pairs are onethree neighbors
+  //          datum = onetwo ID, onetwo ID
+  // owning proc for each datum = onetwo ID % nprocs
 
   nsend = 0;
   for (i = 0; i < nlocal; i++) {
@@ -377,20 +374,19 @@ void Special::onethree_build()
   }
 
   // perform rendezvous operation
-  // each proc owns random subset of atoms
-  // receives all info to form and return their onethree lists
 
   char *buf;
-  int nreturn = comm->rendezvous(nsend,proclist,
-                                 (char *) inbuf,sizeof(PairRvous),
-                                 rendezvous_pairs,buf,sizeof(PairRvous),
+  int nreturn = comm->rendezvous(1,nsend,(char *) inbuf,sizeof(PairRvous),
+                                 0,proclist,
+                                 rendezvous_pairs,0,buf,sizeof(PairRvous),
                                  (void *) this);
   PairRvous *outbuf = (PairRvous *) buf;
 
   memory->destroy(proclist);
   memory->sfree(inbuf);
 
-  // set nspecial[1] and onethree for all owned atoms based on output info
+  // set nspecial[1] and onethree for all owned atoms
+  // based on owned info plus rendezvous output info
   // output datums = pairs of atoms that are 1-3 neighbors
 
   for (i = 0; i < nlocal; i++) {
@@ -434,7 +430,8 @@ void Special::onethree_build()
 }
 
 /* ----------------------------------------------------------------------
-   remove duplicates within each of onetwo, onethree, onefour individually
+   onefour build
+   uses rendezvous comm
 ------------------------------------------------------------------------- */
 
 void Special::onefour_build()
@@ -446,7 +443,6 @@ void Special::onefour_build()
   int nlocal = atom->nlocal;
 
   // nsend = # of my datums to send
-  // include nlocal datums with owner of each atom
 
   int nsend = 0;
   for (i = 0; i < nlocal; i++) {
@@ -462,10 +458,10 @@ void Special::onefour_build()
     memory->smalloc((bigint) nsend*sizeof(PairRvous),"special:inbuf");
 
   // setup input buf to rendezvous comm
-  // input datums = all pairs of onethree and onetwo atoms (they're 1-4 neighbors)
-  // owning proc for each datum = random hash of atomID
-  // one datum for each owned atom: datum = owning proc, atomID
-  // one datum for each onethree/onetwo pair: datum = atomID1, atomID2
+  // datums = pairs of onethree and onetwo partners where onethree is unknown
+  //          these pairs are onefour neighbors
+  //          datum = onetwo ID, onetwo ID
+  // owning proc for each datum = onethree ID % nprocs
 
   nsend = 0;
   for (i = 0; i < nlocal; i++) {
@@ -483,20 +479,19 @@ void Special::onefour_build()
   }
 
   // perform rendezvous operation
-  // each proc owns random subset of atoms
-  // receives all info to form and return their onefour lists
 
   char *buf;
-  int nreturn = comm->rendezvous(nsend,proclist,
-                                 (char *) inbuf,sizeof(PairRvous),
-                                 rendezvous_pairs,buf,sizeof(PairRvous),
+  int nreturn = comm->rendezvous(1,nsend,(char *) inbuf,sizeof(PairRvous),
+                                 0,proclist,
+                                 rendezvous_pairs,0,buf,sizeof(PairRvous),
                                  (void *) this);
   PairRvous *outbuf = (PairRvous *) buf;
 
   memory->destroy(proclist);
   memory->sfree(inbuf);
 
-  // set nspecial[2] and onefour for all owned atoms based on output info
+  // set nspecial[2] and onefour for all owned atoms
+  // based on owned info plus rendezvous output info
   // output datums = pairs of atoms that are 1-4 neighbors
 
   for (i = 0; i < nlocal; i++) {
@@ -780,6 +775,7 @@ void Special::combine()
    trim list of 1-3 neighbors by checking defined angles
    delete a 1-3 neigh if they are not end atoms of a defined angle
      and if they are not 1,3 or 2,4 atoms of a defined dihedral
+   uses rendezvous comm
 ------------------------------------------------------------------------- */
 
 void Special::angle_trim()
@@ -849,6 +845,10 @@ void Special::angle_trim()
       memory->smalloc((bigint) nsend*sizeof(PairRvous),"special:inbuf");
 
     // setup input buf to rendezvous comm
+    // datums = pairs of onetwo partners where either is unknown
+    //          these pairs are onethree neighbors
+    //          datum = onetwo ID, onetwo ID
+    // owning proc for each datum = onetwo ID % nprocs
 
     nsend = 0;
     for (i = 0; i < nlocal; i++) {
@@ -902,14 +902,11 @@ void Special::angle_trim()
     }
 
     // perform rendezvous operation
-    // each proc owns random subset of atoms
-    // func = compute bbox of each body, flag atom closest to geometric center
-    // when done: each atom has atom ID of owning atom of its body
     
     char *buf;
-    int nreturn = comm->rendezvous(nsend,proclist,
-                                   (char *) inbuf,sizeof(PairRvous),
-                                   rendezvous_pairs,buf,sizeof(PairRvous),
+    int nreturn = comm->rendezvous(1,nsend,(char *) inbuf,sizeof(PairRvous),
+                                   0,proclist,
+                                   rendezvous_pairs,0,buf,sizeof(PairRvous),
                                    (void *) this);
     PairRvous *outbuf = (PairRvous *) buf;
 
@@ -931,6 +928,8 @@ void Special::angle_trim()
 	flag[i][j] = 0;
     
     // reset nspecial[1] and onethree for all owned atoms based on output info
+    // based on owned info plus rendezvous output info
+    // output datums = pairs of atoms that are 1-3 neighbors
     
     for (i = 0; i < nlocal; i++) {
       for (j = 0; j < num_angle[i]; j++) {
@@ -1036,8 +1035,9 @@ void Special::angle_trim()
 }
 
 /* ----------------------------------------------------------------------
-   trim list of 1-4 neighbors by checking defined dihedrals
+   trim list of 1-4 neighbors by checking all defined dihedrals
    delete a 1-4 neigh if they are not end atoms of a defined dihedral
+   uses rendezvous comm
 ------------------------------------------------------------------------- */
 
 void Special::dihedral_trim()
@@ -1068,12 +1068,11 @@ void Special::dihedral_trim()
               "  %g = # of 1-4 neighbors before dihedral trim\n",allcount);
   }
 
-  // if dihedrals are defined, rendezvous dihedral 1-4 pairs
+  // if dihedrals are defined, rendezvous the dihedral 1-4 pairs
 
   if (num_dihedral && atom->ndihedrals) {
 
     // nsend = # of my datums to send
-    // latter is only for dihedrals where I own atom2 (newton bond off)
 
     int nsend = 0;
     for (i = 0; i < nlocal; i++) {
@@ -1092,6 +1091,11 @@ void Special::dihedral_trim()
       memory->smalloc((bigint) nsend*sizeof(PairRvous),"special:inbuf");
     
     // setup input buf to rendezvous comm
+    // datums = pairs of onefour atom IDs in a dihedral defined for my atoms
+    //          only dihedrals where I own atom2 (in case newton_bond off)
+    //          datum = atom1 ID and atom4 ID
+    //          send the datum twice, to owner of atom1 ID and atom4 ID
+    // owning procs for each datum = atom1 or atom4 ID % nprocs
 
     nsend = 0;
     for (i = 0; i < nlocal; i++) {
@@ -1117,21 +1121,18 @@ void Special::dihedral_trim()
     }
 
     // perform rendezvous operation
-    // each proc owns random subset of atoms
-    // func = compute bbox of each body, flag atom closest to geometric center
-    // when done: each atom has atom ID of owning atom of its body
     
     char *buf;
-    int nreturn = comm->rendezvous(nsend,proclist,
-                                   (char *) inbuf,sizeof(PairRvous),
-                                   rendezvous_pairs,buf,sizeof(PairRvous),
+    int nreturn = comm->rendezvous(1,nsend,(char *) inbuf,sizeof(PairRvous),
+                                   0,proclist,
+                                   rendezvous_pairs,0,buf,sizeof(PairRvous),
                                    (void *) this);
     PairRvous *outbuf = (PairRvous *) buf;
 
     memory->destroy(proclist);
     memory->sfree(inbuf);
 
-    // flag all onefour atoms to keep
+    // flag all of my onefour IDs to keep
 
     int max = 0;
     for (i = 0; i < nlocal; i++)
@@ -1145,8 +1146,6 @@ void Special::dihedral_trim()
       for (j = 0; j < nspecial[i][2]; j++)
 	flag[i][j] = 0;
     
-    // reset nspecial[2] and onefour for all owned atoms based on output info
-
     for (i = 0; i < nlocal; i++) {
       for (j = 0; j < num_dihedral[i]; j++) {
 	if (tag[i] != dihedral_atom2[i][j]) continue;
@@ -1251,7 +1250,7 @@ int Special::rendezvous_ids(int n, char *inbuf,
   sptr->procowner = procowner;
   sptr->atomIDs = atomIDs;
 
-  // flag = 0: no 2nd irregular comm needed in comm->rendezvous
+  // flag = 0: no second comm needed in rendezvous
   
   flag = 0;
   return 0;
@@ -1272,7 +1271,7 @@ int Special::rendezvous_pairs(int n, char *inbuf,
   Atom *atom = sptr->atom;
   Memory *memory = sptr->memory;
 
-  // clear atom map so it can be here as a hash table
+  // clear atom map so it can be used here as a hash table
   // faster than an STL map for large atom counts
 
   atom->map_clear();