diff --git a/src/relaxng/settings.rnc b/src/relaxng/settings.rnc
index a7b92e1738..092bf9983c 100644
--- a/src/relaxng/settings.rnc
+++ b/src/relaxng/settings.rnc
@@ -27,7 +27,7 @@ element settings {
     (element weight_avg { xsd:double } | attribute weight_avg { xsd:double })?
   }? &
 
-  element energy_grid { ( "nuclide" | "log" | "logarithm" | "logarithmic" ) }? &
+  element energy_grid { ( "nuclide" | "log" | "logarithm" | "logarithmic" | "material-union" | "union" ) }? &
 
   element entropy {
     (element dimension { list { xsd:int+ } } |
diff --git a/src/relaxng/settings.rng b/src/relaxng/settings.rng
index 02dba3688e..10770f80cc 100644
--- a/src/relaxng/settings.rng
+++ b/src/relaxng/settings.rng
@@ -106,6 +106,8 @@
           <value>log</value>
           <value>logarithm</value>
           <value>logarithmic</value>
+          <value>material-union</value>
+          <value>union</value>
         </choice>
       </element>
     </optional>