diff --git a/Basis/ERI.ml b/Basis/ERI.ml
index b7324a2..7e13d3f 100644
--- a/Basis/ERI.ml
+++ b/Basis/ERI.ml
@@ -19,11 +19,19 @@ type t = (float, float64_elt, fortran_layout) Bigarray.Genarray.t
 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
-  boys_function ~maxm t
-  |> Array.mapi (fun m fm ->
-      two_over_sq_pi *. (if m mod 2 = 0 then fm else -.fm) *.
-      (pow exp_pq m) *. (sqrt exp_pq)
-    )
+  let f = two_over_sq_pi *. (sqrt exp_pq) in
+  let result = boys_function ~maxm t in
+  let rec aux accu k = function
+    | 0 -> result.(k) <- result.(k) *. accu
+    | l -> 
+      begin
+        result.(k) <- result.(k) *. accu;
+        let new_accu = -. accu *. exp_pq in
+        aux new_accu (k+1) (l-1)
+      end
+  in
+  aux f 0 maxm;
+  result
 
 
 (** Compute all the integrals of a contracted class *)