diff --git a/doc/fix_shake.txt b/doc/fix_shake.txt
index 6885305d73..28257f6ee2 100644
--- a/doc/fix_shake.txt
+++ b/doc/fix_shake.txt
@@ -210,6 +210,9 @@ 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.
 
+SHAKE or RATTLE should not be used to contrain an angle at 180 degrees
+(e.g. linear CO2 molecule).  This causes numeric difficulties.
+
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