diff --git a/src/colvar/PathMSD.cpp b/src/colvar/PathMSD.cpp
index 63db7e32dd2db26647134e37a3ea5ce9d4262100..fd93c8c82b081fb4264a20615673ed61b850b283 100644
--- a/src/colvar/PathMSD.cpp
+++ b/src/colvar/PathMSD.cpp
@@ -51,6 +51,8 @@ note that NEIGH_STRIDE=4 NEIGH_SIZE=8 control the neighborlist parameter (option
 recommended for performance) and states that the neighbor list will be calculated every 4
 timesteps and consider only the closest 8 member to the actual md snapshots.
 
+In the REFERENCE PDB file the frames must be separated either using END or ENDMDL.
+
 \note
 The implementation of this collective variable and of \ref PROPERTYMAP
 is shared, as well as most input options.