diff --git a/doc/doc2/fix_shake.html b/doc/doc2/fix_shake.html
index 064713a0e7749675fd496015a34d70f22fa7bbbe..9aedcf50ad4e58487747e4c1f65fb0f1680ae6b0 100644
--- a/doc/doc2/fix_shake.html
+++ b/doc/doc2/fix_shake.html
@@ -226,6 +226,9 @@ which can lead to poor energy conservation.  You can test for this in
 your system by running a constant NVE simulation with a particular set
 of SHAKE parameters and monitoring the energy versus time.
 </P>
+<P>SHAKE or RATTLE should not be used to contrain an angle at 180 degrees
+(e.g. linear CO2 molecule).  This causes numeric difficulties.
+</P>
 <P><B>Related commands:</B> none
 </P>
 <P><B>Default:</B> none