diff --git a/env/merlin/spack.yaml b/env/merlin/spack.yaml
index b545cdf..98fd9a3 100644
--- a/env/merlin/spack.yaml
+++ b/env/merlin/spack.yaml
@@ -1,6 +1,6 @@
 spack:
   concretizer:
-    unify: false
+    unify: when_possible
   definitions:
     - Compilers: &Compilers
       - '%clang@12.0.0'