diff --git a/src/compute_sum.cpp b/src/compute_sum.cpp
new file mode 100644
index 0000000000..20443deb5e
--- /dev/null
+++ b/src/compute_sum.cpp
@@ -0,0 +1,156 @@
+/* ----------------------------------------------------------------------
+   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.
+------------------------------------------------------------------------- */
+
+#include "string.h"
+#include "compute_sum.h"
+#include "atom.h"
+#include "modify.h"
+#include "force.h"
+#include "comm.h"
+#include "memory.h"
+#include "error.h"
+
+using namespace LAMMPS_NS;
+
+/* ---------------------------------------------------------------------- */
+
+ComputeSum::ComputeSum(LAMMPS *lmp, int narg, char **arg) :
+  Compute(lmp, narg, arg)
+{
+  if (narg < 4) error->all("Illegal compute sum command");
+
+  // store pre-compute IDs
+
+  npre = narg - 3;
+  id_pre = new char*[npre];
+  for (int i = 0; i < npre; i++) {
+    int iarg = i + 3;
+    int n = strlen(arg[iarg]) + 1;
+    id_pre[i] = new char[n];
+    strcpy(id_pre[i],arg[iarg]);
+  }
+
+  compute = new Compute*[npre];
+
+  // all sub-computes must be peratom
+  // check consistency of sub-computes for scalar & vector output
+
+  int icompute;
+  for (int i = 0; i < npre; i++) {
+    icompute = modify->find_compute(id_pre[i]);
+    if (icompute < 0)
+      error->all("Could not find compute sum/atom pre-compute ID");
+    if (modify->compute[icompute]->peratom_flag == 0)
+      error->all("Compute sum compute is not a per-atom compute");
+  }
+
+  peratom_flag = 0;
+  extensive = 0;
+
+  icompute = modify->find_compute(id_pre[0]);
+  int size = modify->compute[icompute]->size_peratom;
+  if (size == 0) {
+    scalar_flag = 1;
+    vector = NULL;
+  } else {
+    vector_flag = 1;
+    size_vector = size;
+    vector = new double[size_vector];
+  }
+
+  for (int i = 1; i < npre; i++) {
+    icompute = modify->find_compute(id_pre[i]);
+    if (modify->compute[icompute]->size_peratom != size)
+      error->all("Inconsistent sizes of compute sum compute quantities");
+  }
+}
+
+/* ---------------------------------------------------------------------- */
+
+ComputeSum::~ComputeSum()
+{
+  delete [] compute;
+  delete [] vector;
+}
+
+/* ---------------------------------------------------------------------- */
+
+void ComputeSum::init()
+{
+  // set ptrs to Computes used as pre-computes by this compute
+
+  for (int i = 0; i < npre; i++) {
+    int icompute = modify->find_compute(id_pre[i]);
+    if (icompute < 0)
+      error->all("Could not find compute sum/atom pre-compute ID");
+    compute[i] = modify->compute[icompute];
+  }
+}
+
+/* ---------------------------------------------------------------------- */
+
+double ComputeSum::compute_scalar()
+{
+  int i;
+
+  // invoke all the pre-computes
+  // this is the only compute that does this
+  // done b/c pre-computes are per-atom and this compute is not
+
+  for (int icompute = 0; icompute < npre; icompute++)
+    compute[icompute]->compute_peratom();
+
+  // compute scalar quantity by summing over atom scalars
+
+  int *mask = atom->mask;
+  int nlocal = atom->nlocal;
+
+  scalar = 0.0;
+
+  for (int icompute = 0; icompute < npre; icompute++) {
+    double *scalar_atom = compute[icompute]->scalar_atom;
+    for (i = 0; i < nlocal; i++)
+      if (mask[i] & groupbit) scalar += scalar_atom[i];
+  }
+
+  return scalar;
+}
+
+/* ---------------------------------------------------------------------- */
+
+void ComputeSum::compute_vector()
+{
+  int i,j;
+
+  // invoke all the pre-computes
+  // this is the only compute that does this
+  // done b/c pre-computes are per-atom and this compute is not
+
+  for (int icompute = 0; icompute < npre; icompute++)
+    compute[icompute]->compute_peratom();
+
+  // compute vector quantity by summing over atom vectors
+
+  int *mask = atom->mask;
+  int nlocal = atom->nlocal;
+
+  for (j = 0; j < size_vector; j++) vector[j] = 0.0;
+
+  for (int icompute = 0; icompute < npre; icompute++) {
+    double **vector_atom = compute[icompute]->vector_atom;
+    for (i = 0; i < nlocal; i++)
+      if (mask[i] & groupbit) 
+	for (j = 0; j < size_vector; j++)
+	  vector[j] += vector_atom[i][j];
+  }
+}