diff --git a/.merlin b/.merlin
index c5ed8ad..9f51dcc 100644
--- a/.merlin
+++ b/.merlin
@@ -1,4 +1,4 @@
-PKG str unix bigarray lacaml
+PKG str unix bigarray lacaml zarith mpi alcotest
 S .
 S Test
 S Nuclei
diff --git a/CI/Determinant.ml b/CI/Determinant.ml
index c0fa584..6d7c342 100644
--- a/CI/Determinant.ml
+++ b/CI/Determinant.ml
@@ -1,14 +1,18 @@
-type sign_type =
+type phase_type =
 | Plus
 | Minus
 
-type spin_determinant_type = Z.t
+type spin_determinant_type =
+{
+  bitstring : Z.t ;
+  phase  : phase_type;
+}
 
 type determinant_type =
 {
-  alpha : spin_determinant_type ;
-  beta  : spin_determinant_type ;
-  sign  : sign_type;
+  alpha : Z.t ;
+  beta  : Z.t ;
+  phase  : phase_type;
 }
 
 
@@ -20,11 +24,11 @@ let spindet_of_list l =
   | [] -> accu, if nperm mod 2 = 0 then Plus else Minus
   | i :: rest -> 
     let i = pred i in
-    let x = Z.(one lsl i) in
-    let accu = Z.(x lor accu) in
+    let x = Z.(shift_left one i) in
+    let accu = Z.logor accu x in
     let nperm =
       let mask = Z.(x-one) in
-      let r = Z.(accu land mask) in
+      let r = Z.logand accu mask in
       if r = Z.zero then
         nperm
       else
@@ -48,25 +52,25 @@ let rec to_list spindet =
   
 
 let of_lists a b =
-  let alpha, sign_a = 
+  let alpha, phase_a = 
     spindet_of_list a
   in
-  let beta, sign_b = 
+  let beta, phase_b = 
     spindet_of_list b
   in
-  let sign =
-    match sign_a, sign_b with
+  let phase =
+    match phase_a, phase_b with
     | Plus , Plus -> Plus
     | Minus, Minus-> Plus
     | _ -> Minus
   in
-  { alpha ; beta ; sign }
+  { alpha ; beta ; phase }
 
 let alpha det = det.alpha
 let beta  det = det.beta
 
 let sgn det =
-  match det.sign with
+  match det.phase with
   | Plus  ->  1.
   | Minus -> -1.
 
diff --git a/CI/Phase.ml b/CI/Phase.ml
new file mode 100644
index 0000000..d4febb2
--- /dev/null
+++ b/CI/Phase.ml
@@ -0,0 +1,12 @@
+type t =
+| Pos 
+| Neg
+
+let of_nperm nperm =
+  if (nperm land 1) = 1 then Neg
+  else Pos
+
+let pp_phase ppf = function
+  | Pos -> Format.fprintf ppf "@[<h>+1@]"
+  | Neg -> Format.fprintf ppf "@[<h>-1@]"
+
diff --git a/CI/Phase.mli b/CI/Phase.mli
new file mode 100644
index 0000000..5aaeec7
--- /dev/null
+++ b/CI/Phase.mli
@@ -0,0 +1,10 @@
+type t =
+| Pos 
+| Neg
+
+val of_nperm : int -> t
+(** Returns the phase obtained by a given number of permuations. *)
+
+
+(** Formatters *)
+val pp_phase : Format.formatter -> t -> unit
diff --git a/CI/Spindeterminant.ml b/CI/Spindeterminant.ml
new file mode 100644
index 0000000..02b6c66
--- /dev/null
+++ b/CI/Spindeterminant.ml
@@ -0,0 +1,64 @@
+type t =
+  {
+    bitstring : Z.t ;
+    phase     : Phase.t ;
+  }
+
+let of_list l = 
+  let rec aux accu nperm = function
+  | [] -> { bitstring = accu;
+            phase = Phase.of_nperm nperm; }
+  | i :: rest -> 
+    let i = pred i in
+    let x = Z.(shift_left one i) in
+    let accu = Z.logor accu x in
+    let nperm =
+      let mask = Z.(x-one) in
+      let r = Z.logand accu mask in
+      if r = Z.zero then
+        nperm
+      else
+        nperm + (Z.popcount r)
+    in
+    aux accu nperm rest
+  in
+  List.rev l
+  |> aux Z.zero 0
+
+
+(* TODO Phase *)
+let rec to_list spindet =
+  let rec aux accu z =
+    if z <> Z.zero then
+      let element = (Z.(trailing_zeros z)+1) in
+      aux (element::accu) Z.(z land (pred z))
+    else List.rev accu
+  in aux [] spindet
+  
+
+let pp_bitstring ppf bs =
+  String.init (Z.numbits bs) (fun i -> if (Z.testbit bs i) then '+' else '-')
+  |> Format.fprintf ppf "@[<h>%s@]"
+
+let pp_spindet ppf s =
+  Format.fprintf ppf "@[<h>%a %a@]" Phase.pp_phase s.phase pp_bitstring s.bitstring
+
+
+
+let test_case () =
+
+  let test_creation () =
+      let l_a = [ 1 ; 2 ; 3 ; 5 ] in
+      let det = of_list l_a in
+      Alcotest.(check (list int )) "bitstring" (to_list det.bitstring) l_a;
+      Alcotest.(check bool) "phase" (det.phase = Phase.Pos)  true;
+      let l_b = [ 1 ; 3 ; 2 ; 5 ] in
+      let det = of_list l_b in
+      Alcotest.(check (list int )) "bitstring" (to_list det.bitstring) l_a;
+      Alcotest.(check bool) "phase" (det.phase = Phase.Neg)  true;
+  in
+  [
+    "Creation", `Quick, test_creation;
+  ]
+
+  
diff --git a/run_tests.ml b/run_tests.ml
index 6e66624..1b884ab 100644
--- a/run_tests.ml
+++ b/run_tests.ml
@@ -15,6 +15,7 @@ let test_water_dz () =
   Alcotest.run "Water, cc-pVDZ" [
     "AO_Basis", AOBasis.test_case ao_basis;
     "Guess", Guess.test_case ao_basis;
+    "Spindeterminant", Spindeterminant.test_case ();
     "Determinant", Determinant.test_case ();
   ]