diff --git a/Basis/TwoElectronRR.ml b/Basis/TwoElectronRR.ml
index 5a564d8..defbd5b 100644
--- a/Basis/TwoElectronRR.ml
+++ b/Basis/TwoElectronRR.ml
@@ -352,9 +352,7 @@ let contracted_class_shell_pairs ~zero_m ?schwartz_p ?schwartz_q shell_p shell_q
         in
 
         let zero_m_array = 
-          let key = String.concat " " 
-            [ string_of_int maxm ; string_of_float expo_pq_inv ; string_of_float norm_pq_sq ]
-          in
+          let key = (maxm, expo_pq_inv, norm_pq_sq) in
           try
             let result = 
               Hashtbl.find zero_m_cache key
diff --git a/Basis/TwoElectronRRVectorized.ml b/Basis/TwoElectronRRVectorized.ml
index 0ad0df0..48e62af 100644
--- a/Basis/TwoElectronRRVectorized.ml
+++ b/Basis/TwoElectronRRVectorized.ml
@@ -338,9 +338,7 @@ let contracted_class_shell_pairs ~zero_m ?schwartz_p ?schwartz_q shell_p shell_q
                 in
 
                 let zero_m_array = 
-                  let key = String.concat " " 
-                    [ string_of_int 0 ; string_of_float expo_pq_inv ; string_of_float norm_pq_sq ]
-                  in
+                  let key = (0, expo_pq_inv, norm_pq_sq) in
                   try
                     let result = 
                       Hashtbl.find zero_m_cache key
@@ -379,9 +377,7 @@ let contracted_class_shell_pairs ~zero_m ?schwartz_p ?schwartz_q shell_p shell_q
                 in
 
                 let zero_m_array = 
-                  let key = String.concat " " 
-                    [ string_of_int maxm ; string_of_float expo_pq_inv ; string_of_float norm_pq_sq ]
-                  in
+                  let key = (maxm, expo_pq_inv, norm_pq_sq) in
                   try
                     let result = 
                       Hashtbl.find zero_m_cache key