diff --git a/Basis/ERI.ml b/Basis/ERI.ml
index 9d6d6d7..bb6f559 100644
--- a/Basis/ERI.ml
+++ b/Basis/ERI.ml
@@ -12,12 +12,8 @@ open Util
                 and $\phi_q$
 *)
 let zero_m ~maxm ~expo_pq_inv ~norm_pq_sq =
-  let exp_pq =
-    1. /. expo_pq_inv
-  in
-  let t =
-    norm_pq_sq *. exp_pq
-  in
+  let exp_pq = 1. /. expo_pq_inv in
+  let t = norm_pq_sq *. exp_pq in
   boys_function ~maxm t
   |> Array.mapi (fun m fm ->
       two_over_sq_pi *. (if m mod 2 = 0 then fm else -.fm) *.
@@ -34,8 +30,14 @@ type n_cls = { n : int ; cls : Z.t array }
 
 (** Write all integrals to a file with the <ij|kl> convention *)
 let to_file ~filename basis =
+  let to_int_tuple x = 
+    let open Zkey in
+    match to_int_tuple Kind_3 x with
+    | Three x -> x
+    | _ -> assert false
+  in
+
   let oc = open_out filename in
-  let key_array =  Array.make 12 0 in
   for i=0 to (Array.length basis) - 1 do
     print_int basis.(i).Contracted_shell.indice ; print_newline ();
     for j=0 to i do
@@ -46,48 +48,36 @@ let to_file ~filename basis =
             contracted_class basis.(i) basis.(j) basis.(k) basis.(l)
           in
 
-          for i_c = 0 to (Array.length basis.(i).Contracted_shell.powers) - 1 do
-            let x = Zkey.(to_int_array Kind_3 basis.(i).Contracted_shell.powers.(i_c)) in
-            key_array.(0) <- x.(0);
-            key_array.(1) <- x.(1);
-            key_array.(2) <- x.(2);
-            for j_c = 0 to (Array.length basis.(j).Contracted_shell.powers) - 1 do
-              let x = Zkey.(to_int_array Kind_3 basis.(j).Contracted_shell.powers.(j_c)) in
-              key_array.(3) <- x.(0);
-              key_array.(4) <- x.(1);
-              key_array.(5) <- x.(2);
-              for k_c = 0 to (Array.length basis.(k).Contracted_shell.powers) - 1 do
-                let x = Zkey.(to_int_array Kind_3 basis.(k).Contracted_shell.powers.(k_c)) in
-                key_array.(6) <- x.(0);
-                key_array.(7) <- x.(1);
-                key_array.(8) <- x.(2);
-                for l_c = 0 to (Array.length basis.(l).Contracted_shell.powers) - 1 do
-                  let x = Zkey.(to_int_array Kind_3 basis.(l).Contracted_shell.powers.(l_c)) in
-                  key_array.( 9) <- x.(0);
-                  key_array.(10) <- x.(1);
-                  key_array.(11) <- x.(2);
-                  let key = 
-                    Zkey.(of_int_array Kind_12  key_array)
-                  in
-                  let value = 
-                    Zmap.find cls key 
-                  in
-                  if (abs_float value > cutoff) then
-                    Printf.fprintf oc "%4d %4d %4d %4d %20.12e\n"
-                      (basis.(i).Contracted_shell.indice + i_c + 1)
-                      (basis.(k).Contracted_shell.indice + k_c + 1)
-                      (basis.(j).Contracted_shell.indice + j_c + 1)
-                      (basis.(l).Contracted_shell.indice + l_c + 1)
-                      value
-                done
-              done
-            done
-          done;
+          (* Write the data in the output file *)
+          Array.iteri (fun i_c powers_i ->
+              let i_c = basis.(i).Contracted_shell.indice + i_c + 1 in
+              let xi = to_int_tuple powers_i in
+              Array.iteri (fun j_c powers_j ->
+                  let j_c = basis.(j).Contracted_shell.indice + j_c + 1 in
+                  let xj = to_int_tuple powers_j in
+                  Array.iteri (fun k_c powers_k ->
+                      let k_c = basis.(k).Contracted_shell.indice + k_c + 1 in 
+                      let xk = to_int_tuple powers_k in
+                      Array.iteri (fun l_c powers_l ->
+                          let l_c = basis.(l).Contracted_shell.indice + l_c + 1 in
+                          let xl = to_int_tuple powers_l in
+                          let key = 
+                            Zkey.of_int_tuple (Zkey.Twelve (xi,xj,xk,xl))
+                          in
+                          let value = 
+                            Zmap.find cls key 
+                          in
+                          if (abs_float value > cutoff) then
+                            Printf.fprintf oc "%4d %4d %4d %4d %20.12e\n"
+                              i_c k_c j_c l_c value
+                        ) basis.(l).Contracted_shell.powers 
+                    ) basis.(k).Contracted_shell.powers
+                ) basis.(j).Contracted_shell.powers
+            ) basis.(i).Contracted_shell.powers;
         done;
       done;
     done;
-  done
-  ;
+  done;
   close_out oc
 
 (*