-step in which neighbor searching is performed. Since the neighbor
-searching is based on charge groups, charge groups are also the units
-for the domain decomposition. Charge groups are assigned to the cell
-where their center of geometry resides. Before the forces can be
-calculated, the coordinates from some neighboring cells need to be
+step in which neighbor searching is performed. The minimum unit of
+partitioning can be an atom, or a charge group with the (deprecated)
+group cut-off scheme or an update group. An update group is a group
+of atoms that has dependencies during update, which occurs when using
+constraints and/or virtual sites. Thus different update groups can be
+updated independenly. Currently update groups can only be used with at most
+two sequential constraints, which is the case when only constraining
+bonds involving hydrogen atoms. The advantages of update groups are that
+no communication is required in the update and that this allows updating part
+of the system while computing forces for other parts. Atom groups are assigned
+to the cell where their center of geometry resides. Before the forces can
+be calculated, the coordinates from some neighboring cells need to be