diff --git a/modules/mol/mm/src/simulation.cc b/modules/mol/mm/src/simulation.cc
index e6952ff1d69a7b5b0d4289980bece3072288a21d..d338b52e281e59ae49a09b0ac9c70f58075dd5fc 100644
--- a/modules/mol/mm/src/simulation.cc
+++ b/modules/mol/mm/src/simulation.cc
@@ -378,7 +378,7 @@ void Simulation::UpdatePositions(bool enforce_periodic_box){
     throw ost::Error("Num particles in topology and num atoms in entity are not consistent!");
   }
   geom::Vec3List positions = this->GetPositions(enforce_periodic_box, true);
-  ost::mol::XCSEditor ed = ent_.EditXCS();
+  ost::mol::XCSEditor ed = ent_.EditXCS(ost::mol::BUFFERED_EDIT);
   ost::mol::AtomHandleList atom_list = ent_.GetAtomList();
   ost::mol::AtomHandleList::iterator a = atom_list.begin();
   ost::mol::AtomHandleList::iterator ae = atom_list.end();