Do not allow for M2L interactions if either of the cells have single particles

src/cell.c

@@ -6392,6 +6392,10 @@ int cell_can_use_pair_mm(const struct cell *ci, const struct cell *cj,
   const int periodic = s->periodic;
   const double dim[3] = {s->dim[0], s->dim[1], s->dim[2]};
 
+  /* Check for trivial cases */
+  if (ci->grav.count < 2) return 0;
+  if (cj->grav.count < 2) return 0;
+
   /* Recover the multipole information */
   const struct gravity_tensors *const multi_i = ci->grav.multipole;
   const struct gravity_tensors *const multi_j = cj->grav.multipole;