diff --git a/CI/CI.ml b/CI/CI.ml
index 29680b3..e222865 100644
--- a/CI/CI.ml
+++ b/CI/CI.ml
@@ -118,8 +118,11 @@ let make det_space =
   let m_H = lazy (
     let v = Vec.make0 ndet in
     Array.init ndet (fun i -> let ki = det.(i) in
-      Array.iteri (fun j kj ->
-        v.{j+1} <- h_ij mo_basis ki kj) det;
+      Printf.eprintf "%8d / %8d\r%!" i ndet;
+      let j = ref 1 in
+      Ds.determinant_stream det_space
+      |> Stream.iter (fun kj ->
+        v.{!j} <- h_ij mo_basis ki kj ; incr j);
       Vector.sparse_of_vec v)
     |> Matrix.sparse_of_vector_array
   )
diff --git a/CI/Determinant_space.ml b/CI/Determinant_space.ml
index f149c63..3eefe87 100644
--- a/CI/Determinant_space.ml
+++ b/CI/Determinant_space.ml
@@ -1,20 +1,81 @@
+type determinant_storage =
+| Arbitrary of Determinant.t array
+| Spin      of (Spindeterminant.t array * Spindeterminant.t array)
+
 type t = 
 {
   n_alfa       : int ;
   n_beta       : int ;
   mo_class     : MOClass.t ;
   mo_basis     : MOBasis.t ;
-  determinants : Determinant.t array;
+  determinants : determinant_storage;
 }
 
+
 module Ss = Spindeterminant_space
 
 let n_alfa t = t.n_alfa
 let n_beta t = t.n_beta
 let mo_class t = t.mo_class
 let mo_basis t = t.mo_basis
-let determinants t = t.determinants
-let size     t = Array.length t.determinants
+
+
+let size t = 
+  match t.determinants with
+  | Arbitrary a -> Array.length a
+  | Spin (a,b)  ->  (Array.length a) * (Array.length b)
+ 
+
+let determinant_stream t =
+  let imax = size t in
+  match t.determinants with
+  | Arbitrary a -> 
+    Stream.from (fun i ->
+        if i < imax then Some a.(i) else None)
+  | Spin (a,b) -> 
+    let na = Array.length a
+    and nb = Array.length b in
+    let i = ref 0
+    and j = ref 0 in
+    Stream.from (fun k ->
+        if !j < nb then
+          let result = 
+            Determinant.of_spindeterminants a.(!i) b.(!j)
+          in
+          incr i;
+          if !i = na then (i := 0 ; incr j);
+          Some result
+        else
+          None)
+
+
+let determinants t =
+  match t.determinants with
+  | Arbitrary a -> a
+  | Spin (a,b)  -> 
+      let s = determinant_stream t in
+      Array.init (Array.length a * Array.length b) (fun _ ->
+        Stream.next s)
+      (*
+    Array.to_list b
+    |> List.map (fun det_b ->
+        Array.map (fun det_a ->
+            Determinant.of_spindeterminants det_a det_b
+          ) a
+        )
+    |> Array.concat
+    *)
+
+
+let determinant t i =
+  match t.determinants with
+  | Arbitrary a -> a.(i)
+  | Spin (a,b) ->
+    let nb = Array.length b in
+    let k = i / nb in
+    let j = i - k * nb in
+    Determinant.of_spindeterminants a.(j) b.(k)
+
 
 let fci_of_mo_basis ?(frozen_core=true) mo_basis =
   let s = MOBasis.simulation mo_basis in
@@ -30,20 +91,16 @@ let fci_of_mo_basis ?(frozen_core=true) mo_basis =
   let determinants =
     let a = Ss.spin_determinants det_a
     and b = Ss.spin_determinants det_b
-    in
-    let sb = Ss.size det_b in
-    Array.to_list a
-    |> List.map (fun a_i ->
-        Array.init sb (fun j ->
-            Determinant.of_spindeterminants a_i b.(j)) )
-    |> Array.concat
+    in Spin (a,b)
   in
   { n_alfa ; n_beta ; mo_class ; mo_basis ; determinants }
 
 
 let pp_det_space ppf t =
   Format.fprintf ppf "@[<v 2>[ ";
-  Array.iteri (fun i d -> Format.fprintf ppf "@[<v>@[%8d@]@;@[%a@]@]@;" i
-                    (Determinant.pp_det (MOBasis.size (mo_basis t))) d) t.determinants ;
+  let i = ref 0 in
+  determinant_stream t
+  |> Stream.iter (fun d -> Format.fprintf ppf "@[<v>@[%8d@]@;@[%a@]@]@;" !i
+                    (Determinant.pp_det (MOBasis.size (mo_basis t))) d; incr i) ;
   Format.fprintf ppf "]@]"
 
diff --git a/CI/Determinant_space.mli b/CI/Determinant_space.mli
index 31d03d9..c12b456 100644
--- a/CI/Determinant_space.mli
+++ b/CI/Determinant_space.mli
@@ -21,6 +21,9 @@ val mo_basis : t -> MOBasis.t
 val determinants : t -> Determinant.t array
 (** All the determinants belonging to the space. *)
 
+val determinant_stream : t -> Determinant.t Stream.t
+(** All the determinants belonging to the space, as a stream. *)
+
 val size : t -> int
 (** Size of the determinant space *)