slabhi_hi = boxlo + ((nbin-1)/2 + 1)*binsize;

  }

  // make list of nswap atoms with velocity closest to +/- vtarget

  // make 2 lists of up to nswap atoms with velocity closest to +/- vtarget

  // lists are sorted by closeness to vtarget

  // only consider atoms in the bottom/middle slabs

  // map atoms back into periodic box if necessary

  // insert = location in list to insert new atom

  // loop over nswap pairs

  // find 2 global atoms with smallest delta in bottom/top slabs

  // BIG values are for procs with no atom to contribute

  // MINLOC also communicates which procs own them

  // exchange momenta between the 2 particles

  // if I own both particles just swap, else point2point comm of vel,mass

  int ipositive = 0;

  int inegative = 0;

  int nactual = MIN(nnegative,npositive);

  for (m = 0; m < nactual; m++) {

  for (m = 0; m < nswap; m++) {

    if (ipositive < npositive) mine[0].value = pos_delta[ipositive];

    else mine[0].value = BIG;

    if (inegative < nnegative) mine[1].value = neg_delta[inegative];

    MPI_Allreduce(mine,all,2,MPI_DOUBLE_INT,MPI_MINLOC,world);

    if (all[0].value == BIG || all[1].value == BIG) continue;

    if (me == all[0].proc && me == all[1].proc) {

      ipos = pos_index[ipositive++];

      ineg = neg_index[inegative++];