diff --git a/src/vatom/COM.cpp b/src/vatom/COM.cpp
index 45323ef01d902f0e2682939808b237206d854ec6..3edd46e153002ff7ade81ae554c8c3f12ee3d592 100644
--- a/src/vatom/COM.cpp
+++ b/src/vatom/COM.cpp
@@ -39,6 +39,11 @@ an atom list through the label for the COM action that creates it.
 
 For arbitrary weights (e.g. geometric center) see \ref CENTER.
 
+When running with periodic boundary conditions, the user should take care 
+that the atoms in the COM group actually are in the proper periodic image.
+This is typically achieved using the \ref WHOLEMOLECULE action
+before COM calculation.
+
 \par Examples
 
 The following input instructs plumed to print the distance between the
diff --git a/src/vatom/Center.cpp b/src/vatom/Center.cpp
index e73d2f75997e4091275b8fa93320b95884c67c91..6915ff5714aa136c8354cc6aaed27d03eecd1e6b 100644
--- a/src/vatom/Center.cpp
+++ b/src/vatom/Center.cpp
@@ -40,6 +40,11 @@ Notice that the generated virtual atom has charge equal to the sum of the
 charges and mass equal to the sum of the masses. If used with the MASS flag,
 then it provides a result identical to \ref COM.
 
+When running with periodic boundary conditions, the user should take care 
+that the atoms in the COM group actually are in the proper periodic image.
+This is typically achieved using the \ref WHOLEMOLECULE action
+before COM calculation.
+
 \par Examples
 
 \verbatim