diff --git a/src/neigh_shardlow.cpp b/src/neigh_shardlow.cpp
new file mode 100644
index 0000000000..0163e6eb4d
--- /dev/null
+++ b/src/neigh_shardlow.cpp
@@ -0,0 +1,471 @@
+/* ----------------------------------------------------------------------
+   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
+   http://lammps.sandia.gov, Sandia National Laboratories
+   Steve Plimpton, sjplimp@sandia.gov
+
+   Copyright (2003) Sandia Corporation.  Under the terms of Contract
+   DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
+   certain rights in this software.  This software is distributed under
+   the GNU General Public License.
+
+   See the README file in the top-level LAMMPS directory.
+------------------------------------------------------------------------- */
+
+/* ----------------------------------------------------------------------
+   Contributing authors: 
+   James Larentzos and Timothy I. Mattox (Engility Corporation)
+------------------------------------------------------------------------- */
+
+#include "neighbor.h"
+#include "neigh_list.h"
+#include "neigh_request.h"
+#include "atom.h"
+#include "atom_vec.h"
+#include "molecule.h"
+#include "domain.h"
+#include "group.h"
+#include "memory.h"
+#include "error.h"
+#include "update.h"
+
+using namespace LAMMPS_NS;
+
+/* ----------------------------------------------------------------------
+   convert atom coords into the ssa active interaction region number
+------------------------------------------------------------------------- */
+
+int Neighbor::coord2ssa_airnum(double *x)
+{
+  int ix, iy, iz;
+
+  ix = iy = iz = 0;
+  if (x[2] < domain->sublo[2]) iz = -1;
+  if (x[2] > domain->subhi[2]) iz = 1;
+  if (x[1] < domain->sublo[1]) iy = -1;
+  if (x[1] > domain->subhi[1]) iy = 1;
+  if (x[0] < domain->sublo[0]) ix = -1;
+  if (x[0] > domain->subhi[0]) ix = 1;
+
+  if(iz < 0) return 0;
+
+  if(iz == 0){
+    if( iy<0 ) return 0; // bottom left/middle/right
+    if( (iy==0) && (ix<0)  ) return 0; // left atoms
+    if( (iy==0) && (ix==0) ) return 1; // Locally owned atoms
+    if( (iy==0) && (ix>0)  ) return 3; // Right atoms
+    if( (iy>0)  && (ix==0) ) return 2; // Top-middle atoms
+    if( (iy>0)  && (ix!=0) ) return 4; // Top-right and top-left atoms
+  } else if(iz > 0) {
+    if((ix==0) && (iy==0)) return 5; // Back atoms
+    if((ix==0) && (iy!=0)) return 6; // Top-back and bottom-back atoms
+    if((ix!=0) && (iy==0)) return 7; // Left-back and right-back atoms
+    if((ix!=0) && (iy!=0)) return 8; // Back corner atoms
+  }
+
+  return 0;
+}
+
+/* ----------------------------------------------------------------------
+ *    assign owned and ghost atoms their ssa active interaction region numbers
+ *    Called in the pre_neighbor and setup_pre_neighbor fix stages
+------------------------------------------------------------------------- */
+
+void Neighbor::assign_ssa_airnums()
+{
+  int i,ibin;
+  double **x = atom->x;
+  int *mask = atom->mask;
+  int nlocal = atom->nlocal;
+  int nall = nlocal + atom->nghost;
+
+  if (nall > len_ssa_airnum) {
+    len_ssa_airnum = nall;
+    memory->destroy(ssa_airnum);
+    memory->create(ssa_airnum,len_ssa_airnum,"ssa_airnum");
+  }
+
+  // bin in reverse order so linked list will be in forward order
+
+  if (includegroup) {
+    int bitmask = group->bitmask[includegroup];
+    for (i = nall-1; i >= nlocal; i--) {
+      if (mask[i] & bitmask) {
+        ibin = coord2ssa_airnum(x[i]);
+      } else {
+        ibin = 0;
+      }
+      ssa_airnum[i] = ibin;
+    }
+    // All the local excluded atoms are in the zero airnum
+    ibin = 0;
+    for (i = atom->nlocal-1; i >= atom->nfirst; i--) {
+      ssa_airnum[i] = ibin;
+    }
+  } else {
+    for (i = nall-1; i >= nlocal; i--) {
+      ibin = coord2ssa_airnum(x[i]);
+      ssa_airnum[i] = ibin;
+    }
+  }
+
+  // All the local included atoms are in the same airnum (#1)
+  if (i >= 0) {
+    ibin = coord2ssa_airnum(x[i]);
+    do {
+      ssa_airnum[i] = ibin;
+    } while (--i >= 0);
+  }
+}
+
+/* ----------------------------------------------------------------------
+   routines to create a stencil = list of bin offsets
+   stencil = bins whose closest corner to central bin is within cutoff
+   sx,sy,sz = bin bounds = furthest the stencil could possibly extend
+   3d creates xyz stencil, 2d creates xy stencil
+   for half list with newton on:
+     stencil is bins to the "upper right" of central bin
+     stencil does not include self
+------------------------------------------------------------------------- */
+
+/* ---------------------------------------------------------------------- */
+
+void Neighbor::stencil_half_bin_2d_ssa(NeighList *list,
+                                   int sx, int sy, int sz)
+{
+  int i,j;
+  int *stencil = list->stencil;
+  int nstencil = 0;
+
+  for (j = 0; j <= sy; j++)
+    for (i = -sx; i <= sx; i++)
+      if (j > 0 || (j == 0 && i > 0))
+        if (bin_distance(i,j,0) < cutneighmaxsq)
+          stencil[nstencil++] = j*mbinx + i;
+
+  list->nstencil = nstencil;
+
+  // Now include additional bins for AIR ghosts only
+  for (j = -sy; j <= 0; j++)
+    for (i = -sx; i <= sx; i++) {
+      if (j == 0 && i > 0) continue;
+      if (bin_distance(i,j,0) < cutneighmaxsq)
+        stencil[nstencil++] = j*mbinx + i;
+    }
+
+  while (nstencil < list->maxstencil) {
+    stencil[nstencil++] = INT_MAX;
+  }
+}
+
+/* ---------------------------------------------------------------------- */
+
+void Neighbor::stencil_half_bin_3d_ssa(NeighList *list,
+                                   int sx, int sy, int sz)
+{
+  int i,j,k;
+  int *stencil = list->stencil;
+  int nstencil = 0;
+
+  for (k = 0; k <= sz; k++)
+    for (j = -sy; j <= sy; j++)
+      for (i = -sx; i <= sx; i++)
+        if (k > 0 || j > 0 || (j == 0 && i > 0))
+          if (bin_distance(i,j,k) < cutneighmaxsq)
+            stencil[nstencil++] = k*mbiny*mbinx + j*mbinx + i;
+
+  list->nstencil = nstencil;
+
+  // Now include additional bins for AIR ghosts only
+  for (k = -sz; k < 0; k++)
+    for (j = -sy; j <= sy; j++)
+      for (i = -sx; i <= sx; i++)
+        if (bin_distance(i,j,k) < cutneighmaxsq)
+          stencil[nstencil++] = k*mbiny*mbinx + j*mbinx + i;
+  k = 0; // skip already included bins at k == 0
+  for (j = -sy; j <= 0; j++)
+    for (i = -sx; i <= sx; i++) {
+      if (j == 0 && i > 0) continue;
+      if (bin_distance(i,j,k) < cutneighmaxsq)
+        stencil[nstencil++] = k*mbiny*mbinx + j*mbinx + i;
+    }
+
+  while (nstencil < list->maxstencil) {
+    stencil[nstencil++] = INT_MAX;
+  }
+}
+
+/* ----------------------------------------------------------------------
+   build half list from full list for use by Shardlow Spliting Algorithm
+   pair stored once if i,j are both owned and i < j
+   if j is ghost, only store if j coords are "above and to the right" of i
+   works if full list is a skip list
+------------------------------------------------------------------------- */
+
+void Neighbor::half_from_full_newton_ssa(NeighList *list)
+{
+  int i,j,ii,jj,n,jnum,joriginal;
+  int *neighptr,*jlist;
+
+  int nlocal = atom->nlocal;
+
+  int *ilist = list->ilist;
+  int *numneigh = list->numneigh;
+  int **firstneigh = list->firstneigh;
+  MyPage<int> *ipage = list->ipage;
+
+  int *ilist_full = list->listfull->ilist;
+  int *numneigh_full = list->listfull->numneigh;
+  int **firstneigh_full = list->listfull->firstneigh;
+  int inum_full = list->listfull->inum;
+
+  int inum = 0;
+  ipage->reset();
+
+  // loop over parent full list
+
+  for (ii = 0; ii < inum_full; ii++) {
+    n = 0;
+    neighptr = ipage->vget();
+
+    i = ilist_full[ii];
+
+    // loop over full neighbor list
+
+    jlist = firstneigh_full[i];
+    jnum = numneigh_full[i];
+
+    for (jj = 0; jj < jnum; jj++) {
+      joriginal = jlist[jj];
+      j = joriginal & NEIGHMASK;
+      if (j < nlocal) {
+        if (i > j) continue;
+      } else {
+        if (ssa_airnum[j] <= 0) continue;
+      }
+      neighptr[n++] = joriginal;
+    }
+
+    ilist[inum++] = i;
+    firstneigh[i] = neighptr;
+    numneigh[i] = n;
+    ipage->vgot(n);
+    if (ipage->status())
+      error->one(FLERR,"Neighbor list overflow, boost neigh_modify one");
+  }
+
+  list->inum = inum;
+}
+
+/* ----------------------------------------------------------------------
+   for Shardlow Spliting Algorithm:
+   binned neighbor list construction with full Newton's 3rd law
+   each owned atom i checks its own bin and other bins in Newton stencil
+   every pair stored exactly once by some processor
+------------------------------------------------------------------------- */
+
+void Neighbor::half_bin_newton_ssa(NeighList *list)
+{
+  int i,j,k,n,itype,jtype,ibin,which,imol,iatom,moltemplate;
+  tagint tagprev;
+  double xtmp,ytmp,ztmp,delx,dely,delz,rsq;
+  int *neighptr;
+
+  double **x = atom->x;
+  int *type = atom->type;
+  int *mask = atom->mask;
+  tagint *tag = atom->tag;
+  tagint *molecule = atom->molecule;
+  tagint **special = atom->special;
+  int **nspecial = atom->nspecial;
+  int nlocal = atom->nlocal;
+  int nall = nlocal + atom->nghost;
+
+  int *molindex = atom->molindex;
+  int *molatom = atom->molatom;
+  Molecule **onemols = atom->avec->onemols;
+  int molecular = atom->molecular;
+  if (molecular == 2) moltemplate = 1;
+  else moltemplate = 0;
+
+  int *ilist = list->ilist;
+  int *numneigh = list->numneigh;
+  int **firstneigh = list->firstneigh;
+  int nstencil = list->nstencil;
+  int maxstencil = list->maxstencil;
+  int *stencil = list->stencil;
+  MyPage<int> *ipage = list->ipage;
+
+  int inum = 0;
+
+/* ----------------------------------------------------------------------
+   bin owned and ghost atoms for use by Shardlow Splitting Algorithm
+    exclude ghost atoms that are not in the Active Interaction Regions (AIR)
+------------------------------------------------------------------------- */
+
+  if (mbins > maxhead_ssa) {
+    maxhead_ssa = mbins;
+    memory->destroy(gbinhead_ssa);
+    memory->destroy(binhead_ssa);
+    memory->create(binhead_ssa,maxhead_ssa,"binhead_ssa");
+    memory->create(gbinhead_ssa,maxhead_ssa,"gbinhead_ssa");
+  }
+  for (i = 0; i < mbins; i++) {
+    gbinhead_ssa[i] = -1;
+    binhead_ssa[i] = -1;
+  }
+
+  if (maxbin > maxbin_ssa) {
+    maxbin_ssa = maxbin;
+    memory->destroy(bins_ssa);
+    memory->create(bins_ssa,maxbin_ssa,"bins_ssa");
+  }
+
+  // bin in reverse order so linked list will be in forward order
+
+  if (includegroup) {
+    int bitmask = group->bitmask[includegroup];
+    for (i = nall-1; i >= nlocal; i--) {
+      if (ssa_airnum[i] <= 0) continue; // skip ghost atoms not in AIR
+      if (mask[i] & bitmask) {
+        ibin = coord2bin(x[i]);
+        bins_ssa[i] = gbinhead_ssa[ibin];
+        gbinhead_ssa[ibin] = i;
+      }
+    }
+    nlocal = atom->nfirst; // This is important for the code that follows!
+  } else {
+    for (i = nall-1; i >= nlocal; i--) {
+      if (ssa_airnum[i] <= 0) continue; // skip ghost atoms not in AIR
+      ibin = coord2bin(x[i]);
+      bins_ssa[i] = gbinhead_ssa[ibin];
+      gbinhead_ssa[ibin] = i;
+    }
+  }
+  for (i = nlocal-1; i >= 0; i--) {
+    ibin = coord2bin(x[i]);
+    bins_ssa[i] = binhead_ssa[ibin];
+    binhead_ssa[ibin] = i;
+  }
+
+  ipage->reset();
+
+  // loop over owned atoms, storing half of the neighbors
+
+  for (i = 0; i < nlocal; i++) {
+    n = 0;
+    neighptr = ipage->vget();
+
+    itype = type[i];
+    xtmp = x[i][0];
+    ytmp = x[i][1];
+    ztmp = x[i][2];
+    if (moltemplate) {
+      imol = molindex[i];
+      iatom = molatom[i];
+      tagprev = tag[i] - iatom - 1;
+    }
+
+    // loop over rest of local atoms in i's bin
+    // just store them, since j is beyond i in linked list
+
+    for (j = bins_ssa[i]; j >= 0; j = bins_ssa[j]) {
+
+      jtype = type[j];
+      if (exclude && exclusion(i,j,itype,jtype,mask,molecule)) continue;
+
+      delx = xtmp - x[j][0];
+      dely = ytmp - x[j][1];
+      delz = ztmp - x[j][2];
+      rsq = delx*delx + dely*dely + delz*delz;
+
+      if (rsq <= cutneighsq[itype][jtype]) {
+        if (molecular) {
+          if (!moltemplate)
+            which = find_special(special[i],nspecial[i],tag[j]);
+          else if (imol >= 0)
+            which = find_special(onemols[imol]->special[iatom],
+                                 onemols[imol]->nspecial[iatom],
+                                 tag[j]-tagprev);
+          else which = 0;
+          if (which == 0) neighptr[n++] = j;
+          else if (domain->minimum_image_check(delx,dely,delz))
+            neighptr[n++] = j;
+          else if (which > 0) neighptr[n++] = j ^ (which << SBBITS);
+        } else neighptr[n++] = j;
+      }
+    }
+
+    ibin = coord2bin(x[i]);
+
+    // loop over all local atoms in other bins in "half" stencil
+    for (k = 0; k < nstencil; k++) {
+      for (j = binhead_ssa[ibin+stencil[k]]; j >= 0; j = bins_ssa[j]) {
+
+        jtype = type[j];
+        if (exclude && exclusion(i,j,itype,jtype,mask,molecule)) continue;
+
+        delx = xtmp - x[j][0];
+        dely = ytmp - x[j][1];
+        delz = ztmp - x[j][2];
+        rsq = delx*delx + dely*dely + delz*delz;
+
+        if (rsq <= cutneighsq[itype][jtype]) {
+          if (molecular) {
+            if (!moltemplate)
+              which = find_special(special[i],nspecial[i],tag[j]);
+            else if (imol >= 0)
+              which = find_special(onemols[imol]->special[iatom],
+                                   onemols[imol]->nspecial[iatom],
+                                   tag[j]-tagprev);
+            else which = 0;
+            if (which == 0) neighptr[n++] = j;
+            else if (domain->minimum_image_check(delx,dely,delz))
+              neighptr[n++] = j;
+            else if (which > 0) neighptr[n++] = j ^ (which << SBBITS);
+          } else neighptr[n++] = j;
+        }
+      }
+    }
+
+    // loop over AIR ghost atoms in all bins in "full" stencil
+    // Note: the non-AIR ghost atoms have already been filtered out
+    for (k = 0; k < maxstencil; k++) {
+      if (stencil[k] > mbins) break; /* Check if ghost stencil bins are exhausted */
+      for (j = gbinhead_ssa[ibin+stencil[k]]; j >= 0; j = bins_ssa[j]) {
+
+        jtype = type[j];
+        if (exclude && exclusion(i,j,itype,jtype,mask,molecule)) continue;
+
+        delx = xtmp - x[j][0];
+        dely = ytmp - x[j][1];
+        delz = ztmp - x[j][2];
+        rsq = delx*delx + dely*dely + delz*delz;
+
+        if (rsq <= cutneighsq[itype][jtype]) {
+          if (molecular) {
+            if (!moltemplate)
+              which = find_special(special[i],nspecial[i],tag[j]);
+            else if (imol >= 0)
+              which = find_special(onemols[imol]->special[iatom],
+                                   onemols[imol]->nspecial[iatom],
+                                   tag[j]-tagprev);
+            else which = 0;
+            if (which == 0) neighptr[n++] = j;
+            else if (domain->minimum_image_check(delx,dely,delz))
+              neighptr[n++] = j;
+            else if (which > 0) neighptr[n++] = j ^ (which << SBBITS);
+          } else neighptr[n++] = j;
+        }
+      }
+    }
+
+    ilist[inum++] = i;
+    firstneigh[i] = neighptr;
+    numneigh[i] = n;
+    ipage->vgot(n);
+    if (ipage->status())
+      error->one(FLERR,"Neighbor list overflow, boost neigh_modify one");
+  }
+
+  list->inum = inum;
+}
diff --git a/src/neighbor.cpp b/src/neighbor.cpp
index db28f6d309..e9ee23e37f 100644
--- a/src/neighbor.cpp
+++ b/src/neighbor.cpp
@@ -162,7 +162,6 @@ Neighbor::Neighbor(LAMMPS *lmp) : Pointers(lmp)
   improperlist = NULL;
 
   copymode = 0;
-  last_binning_timestep = -1;
 }
 
 /* ---------------------------------------------------------------------- */
@@ -2015,11 +2014,6 @@ void Neighbor::bin_atoms()
 {
   int i,ibin;
 
-  // NOTE: added for USER-DPD, why do we need this?
-
-  //if (last_binning_timestep == update->ntimestep) return;
-  //last_binning_timestep = update->ntimestep;
-
   for (i = 0; i < mbins; i++) binhead[i] = -1;
 
   // bin in reverse order so linked list will be in forward order
diff --git a/src/neighbor.h b/src/neighbor.h
index 8059e699bf..deba569916 100644
--- a/src/neighbor.h
+++ b/src/neighbor.h
@@ -71,7 +71,7 @@ class Neighbor : protected Pointers {
 
   int cluster_check;               // 1 if check bond/angle/etc satisfies minimg
 
-  // USER-DPD package - better to make this private?
+  // USER-DPD package
 
   int *ssa_airnum;              // AIR number of each atom for SSA in USER-DPD
 
@@ -121,7 +121,6 @@ class Neighbor : protected Pointers {
 
   int binatomflag;                 // bin atoms or not when build neigh list
                                    // turned off by build_one()
-  bigint last_binning_timestep;	   // last step neighbor binning was done
 
   int nbinx,nbiny,nbinz;           // # of global bins
   int *bins;                       // ptr to next atom in each bin
@@ -180,7 +179,12 @@ class Neighbor : protected Pointers {
 
   // USER-DPD package
 
-  int len_ssa_airnum;           // length of ssa_airnum array
+  int len_ssa_airnum;        // length of ssa_airnum array
+  int *bins_ssa;             // ptr to next atom in each bin used by SSA
+  int maxbin_ssa;            // size of bins array used by SSA
+  int *binhead_ssa;          // ptr to 1st atom in each bin used by SSA
+  int *gbinhead_ssa;         // ptr to 1st ghost atom in each bin used by SSA
+  int maxhead_ssa;           // size of binhead array used by SSA
 
   // methods
 
@@ -320,13 +324,13 @@ class Neighbor : protected Pointers {
 
   // SSA neighboring for USER-DPD
 
-  //int coord2ssa_airnum(double *);  // map atom coord to an AIR number
-  //void assign_ssa_airnums();       // set ssa_airnum values
+  int coord2ssa_airnum(double *);  // map atom coord to an AIR number
+  void assign_ssa_airnums();       // set ssa_airnum values
 
-  void half_bin_newton_ssa(NeighList *) {}
-  void half_from_full_newton_ssa(class NeighList *) {}
-  void stencil_half_bin_2d_ssa(class NeighList *, int, int, int) {}
-  void stencil_half_bin_3d_ssa(class NeighList *, int, int, int) {}
+  void half_bin_newton_ssa(NeighList *);
+  void half_from_full_newton_ssa(class NeighList *);
+  void stencil_half_bin_2d_ssa(class NeighList *, int, int, int);
+  void stencil_half_bin_3d_ssa(class NeighList *, int, int, int);
 
   // find_special: determine if atom j is in special list of atom i
   // if it is not, return 0