The serial code

void MyCoordination::calculate() {
  double ncoord = 0.;
  auto pos = getPositions();
  const unsigned nn = pos.size();
  Vector distance;
  for (unsigned int i0 = 0; i0 < nn; ++i0) {
    for (unsigned int i1 = 0; i1 < nn; ++i1) {
      if (i0 == i1) {continue;}
      distance = delta(getPosition(i0), getPosition(i1));
      ncoord += (distance.modulo() < R_0) ? 1 : 0;
    }
  }
  setValue(ncoord);
}

the workflow is:

get the positions
initialize ncoord to zero
do a double loop calculating the distance between each possible couple of atoms and increment ncoord each time the distance is less than R_0 (avoid calculating the distance from self)

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The serial code