diff --git a/Basis/ERI.ml b/Basis/ERI.ml
index a6782c9..c3dbefa 100644
--- a/Basis/ERI.ml
+++ b/Basis/ERI.ml
@@ -89,12 +89,22 @@ let to_file ~filename basis =
             let
               shell_q = shell_pairs.(k).(l)
             in
+            let swap =
+              Array.length shell_q < Array.length shell_p 
+            in
+
             (* Compute all the integrals of the class *)
             let cls =
-              if Array.length shell_q < 2 then
-                contracted_class_shell_pairs ~schwartz_p ~schwartz_q shell_p shell_q 
+              if swap then
+                if Array.length shell_p < 2 then
+                  contracted_class_shell_pairs ~schwartz_p:schwartz_q ~schwartz_q:schwartz_p shell_q shell_p 
+                else
+                  contracted_class_shell_pairs_vec ~schwartz_p:schwartz_q ~schwartz_q:schwartz_p shell_q shell_p 
               else
-                contracted_class_shell_pairs_vec ~schwartz_p ~schwartz_q shell_p shell_q 
+                if Array.length shell_q < 2 then
+                  contracted_class_shell_pairs ~schwartz_p ~schwartz_q shell_p shell_q 
+                else
+                  contracted_class_shell_pairs_vec ~schwartz_p ~schwartz_q shell_p shell_q 
             in
 
             (* Write the data in the output file *)
@@ -111,7 +121,10 @@ let to_file ~filename basis =
                             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))
+                              if swap then
+                                Zkey.of_int_tuple (Zkey.Twelve (xk,xl,xi,xj))
+                              else
+                                Zkey.of_int_tuple (Zkey.Twelve (xi,xj,xk,xl))
                             in
                             let value = 
                               Zmap.find cls key