diff --git a/CI/CI.ml b/CI/CI.ml
index bef7328..d7476be 100644
--- a/CI/CI.ml
+++ b/CI/CI.ml
@@ -637,12 +637,14 @@ let second_order_sum2 { det_space ; m_H ; m_S2 ; eigensystem ; n_states }
 
 
 let is_internal det_space =
+    let mo_class      = DeterminantSpace.mo_class det_space in
+    let numbits = Array.length @@ MOClass.mo_class_array mo_class in
     let m l =
       List.fold_left (fun accu i ->
-          let j = i-1 in Bitstring.(logor accu (shift_left_one j))
-        ) Bitstring.zero l
+          let j = i-1 in
+          Bitstring.logor accu (Bitstring.shift_left_one numbits j)
+        ) (Bitstring.zero numbits) l
     in
-    let mo_class      = DeterminantSpace.mo_class det_space in
     let active_mask   = m (MOClass.active_mos   mo_class) in
     let occ_mask      = m (MOClass.core_mos     mo_class) in
     let inactive_mask = m (MOClass.inactive_mos mo_class) in
diff --git a/CI/Determinant.ml b/CI/Determinant.ml
index 04bce94..d932db1 100644
--- a/CI/Determinant.ml
+++ b/CI/Determinant.ml
@@ -14,10 +14,10 @@ let alfa t = t.alfa
 
 let beta  t = t.beta 
 
-let vac =
+let vac n =
   {
-    alfa = Spindeterminant.vac ;
-    beta = Spindeterminant.vac ;
+    alfa = Spindeterminant.vac n;
+    beta = Spindeterminant.vac n;
   }
 
 let phase t = 
@@ -55,9 +55,9 @@ let degree t t' =
   (degree_alfa t t') + (degree_beta t t')
 
 
-let of_lists a b =
-  let alfa = Spindeterminant.of_list a 
-  and beta  = Spindeterminant.of_list b
+let of_lists n a b =
+  let alfa = Spindeterminant.of_list n a 
+  and beta  = Spindeterminant.of_list n b
   in of_spindeterminants alfa beta
 
 
@@ -103,7 +103,7 @@ let test_case () =
   let test_creation () =
       let l_a = [ 1 ; 2 ; 3 ; 5 ; 64 ]
       and l_b = [ 2 ; 3 ; 5 ; 65 ] in
-      let det = of_lists l_a l_b in
+      let det = of_lists 66 l_a l_b in
       let z_a = alfa det
       and z_b = beta  det in
       Alcotest.(check (list int )) "alfa" (Spindeterminant.to_list z_a) l_a;
@@ -114,19 +114,19 @@ let test_case () =
   let test_phase () =
       let l_a = [ 1 ; 2 ; 3 ; 64 ; 5 ]
       and l_b = [ 2 ; 3 ; 5 ; 65 ] in
-      let det = of_lists l_a l_b in
+      let det = of_lists 66 l_a l_b in
       Alcotest.(check bool) "phase" (phase det = Phase.Neg)  true;
       let l_a = [ 1 ; 2 ; 3 ; 64 ; 5 ]
       and l_b = [ 3 ; 2 ; 5 ; 65 ] in
-      let det = of_lists l_a l_b in
+      let det = of_lists 66 l_a l_b in
       Alcotest.(check bool) "phase" (phase det = Phase.Pos)  true;
       let l_a = [ 1 ; 3 ; 2 ; 64 ; 5 ]
       and l_b = [ 3 ; 2 ; 5 ; 65 ] in
-      let det = of_lists l_a l_b in
+      let det = of_lists 66 l_a l_b in
       Alcotest.(check bool) "phase" (phase det = Phase.Neg)  true;
       let l_a = [ 1 ; 3 ; 2 ; 64 ; 5 ]
       and l_b = [ 3 ; 2 ; 65 ; 5 ] in
-      let det = of_lists l_a l_b in
+      let det = of_lists 66 l_a l_b in
       Alcotest.(check bool) "phase" (phase det = Phase.Pos)  true;
   in
 
@@ -134,22 +134,22 @@ let test_case () =
       let det =
         let open Spin in
         creation Alfa 1 @@ creation Alfa 3 @@ creation Alfa 2 @@ creation Alfa 5 @@ 
-        creation Beta 1 @@ creation Beta 3 @@ creation Beta 4 @@ creation Beta 5 @@ vac
+        creation Beta 1 @@ creation Beta 3 @@ creation Beta 4 @@ creation Beta 5 @@ vac 10
       in
       Alcotest.(check bool) "creation 1" true
-        (det = of_lists [ 1 ; 3 ; 2 ; 5 ] [1 ; 3 ; 4 ; 5 ] );
+        (det = of_lists 10 [ 1 ; 3 ; 2 ; 5 ] [1 ; 3 ; 4 ; 5 ] );
 
       let det' =
         single_excitation Spin.Alfa 3 6 det
       in
       Alcotest.(check bool) "single_exc 1" true
-        (det' = of_lists [ 1 ; 6 ; 2 ; 5 ] [1 ; 3 ; 4 ; 5 ] );
+        (det' = of_lists 10 [ 1 ; 6 ; 2 ; 5 ] [1 ; 3 ; 4 ; 5 ] );
 
       let det' =
         single_excitation Spin.Beta  3 6 det
       in
       Alcotest.(check bool) "single_exc 2" true
-        (det' = of_lists [ 1 ; 3 ; 2 ; 5 ] [1 ; 6 ; 4 ; 5 ] );
+        (det' = of_lists 10 [ 1 ; 3 ; 2 ; 5 ] [1 ; 6 ; 4 ; 5 ] );
 
       let det' =
         single_excitation Spin.Alfa 4 6 det
@@ -164,26 +164,26 @@ let test_case () =
       let det' =
         double_excitation Spin.Alfa 3 6 Spin.Alfa 2 7 det
       in
-      let det'' = of_lists [ 1 ; 6 ; 7 ; 5 ] [1 ; 3 ; 4 ; 5 ] in
+      let det'' = of_lists 10 [ 1 ; 6 ; 7 ; 5 ] [1 ; 3 ; 4 ; 5 ] in
       Alcotest.(check bool) "double_exc 1" true (det' = det'');
 
       let det' =
         double_excitation Spin.Beta  3 6 Spin.Beta 5 7  det
       in
       Alcotest.(check bool) "double_exc 2" true
-        (det' = of_lists [ 1 ; 3 ; 2 ; 5 ] [1 ; 6 ; 4 ; 7 ] );
+        (det' = of_lists 10 [ 1 ; 3 ; 2 ; 5 ] [1 ; 6 ; 4 ; 7 ] );
 
       let det' =
         double_excitation Spin.Alfa 3 6 Spin.Beta  5 7 det
       in
       Alcotest.(check bool) "double_exc 3" true
-        (det' = of_lists [ 1 ; 6 ; 2 ; 5 ] [1 ; 3 ; 4 ; 7 ] );
+        (det' = of_lists 10 [ 1 ; 6 ; 2 ; 5 ] [1 ; 3 ; 4 ; 7 ] );
 
       let det' =
         double_excitation Spin.Beta 5 7 Spin.Alfa 3 6 det
       in
       Alcotest.(check bool) "double_exc 4" true
-        (det' = of_lists [ 1 ; 6 ; 2 ; 5 ] [1 ; 3 ; 4 ; 7 ] );
+        (det' = of_lists 10 [ 1 ; 6 ; 2 ; 5 ] [1 ; 3 ; 4 ; 7 ] );
 
       let det' =
         double_excitation Spin.Alfa 4 6 Spin.Alfa 2 7 det
diff --git a/CI/Determinant.mli b/CI/Determinant.mli
index 2c75204..aa0836f 100644
--- a/CI/Determinant.mli
+++ b/CI/Determinant.mli
@@ -29,8 +29,9 @@ val is_none : t -> bool
 
 (** {1 Second quantization operators} *)
 
-val vac : t
-(** Vacuum state, [vac = Some ]{% $|\rangle$ %}  *)
+val vac : int -> t
+(** Vacuum state, [vac = Some ]{% $|\rangle$ %}. The integer parameter is the size of the
+    MO basis set. *)
 
 val creation : Spin.t -> particle -> t -> t
 (** [creation spin p] is the creation operator {% $a^\dagger_p$ %}. *)
@@ -65,8 +66,9 @@ val of_spindeterminants : Spindeterminant.t -> Spindeterminant.t -> t
     [Spindeterminant.t].
 *)
 
-val of_lists : int list -> int list -> t
-(** Creates a Slater determinant from a two lists of orbital indices.  *)
+val of_lists : int -> int list -> int list -> t
+(** Creates a Slater determinant from a two lists of orbital indices.
+    The integer parameter is the size of the MO basis set. *)
 
 val negate_phase : t -> t
 (** Returns the same determinant with the phase negated. *)
diff --git a/CI/Excitation.ml b/CI/Excitation.ml
index e9854b8..6b154da 100644
--- a/CI/Excitation.ml
+++ b/CI/Excitation.ml
@@ -18,6 +18,7 @@ let single_of_spindet t t' =
   and d' = Spindeterminant.bitstring t'
   in
   let tmp = Bitstring.logxor d d' in
+  let shift_left_one = Bitstring.(shift_left_one (numbits tmp)) in
   let hole_z     = Bitstring.logand (Spindeterminant.bitstring t ) tmp
   and particle_z = Bitstring.logand (Spindeterminant.bitstring t') tmp
   in
@@ -32,8 +33,8 @@ let single_of_spindet t t' =
   let mask =
     let h = high-1 in
     let l = low in
-    let mask_up = Bitstring.shift_left_one h |> Bitstring.minus_one
-    and mask_dn = Bitstring.plus_one @@ Bitstring.lognot (Bitstring.shift_left_one l) 
+    let mask_up = shift_left_one h |> Bitstring.minus_one
+    and mask_dn = Bitstring.plus_one @@ Bitstring.lognot (shift_left_one l) 
     in Bitstring.logand mask_up mask_dn
   in
   let phase = 
@@ -150,7 +151,7 @@ let test_case () =
   let test_single () =
     let l_a = [ 1 ; 2 ; 3 ; 5 ; 64 ]
     and l_b = [ 2 ; 3 ; 5 ; 65 ] in
-    let det1 = Determinant.of_lists l_a l_b in
+    let det1 = Determinant.of_lists 66 l_a l_b in
     let det2 = Determinant.single_excitation Spin.Alfa 3 7 det1 in 
     let t = single_of_det det1 det2 in
     Alcotest.(check bool) "single 1" true (t = Single (Phase.Pos, { hole=3 ; particle=7 ; spin=Spin.Alfa}) );
@@ -171,7 +172,7 @@ let test_case () =
   let test_double () =
     let l_a = [ 1 ; 2 ; 3 ; 5 ; 64 ]
     and l_b = [ 2 ; 3 ; 5 ; 65 ] in
-    let det1 = Determinant.of_lists l_a l_b in
+    let det1 = Determinant.of_lists 66 l_a l_b in
     let det2 = Determinant.double_excitation Spin.Alfa 3 7 Spin.Alfa 2 6 det1 in 
     let t = double_of_det det1 det2 in
     Alcotest.(check bool) "double 1" true
diff --git a/CI/F12CI.ml b/CI/F12CI.ml
index 9143dc7..781ce40 100644
--- a/CI/F12CI.ml
+++ b/CI/F12CI.ml
@@ -64,18 +64,29 @@ let f_ij mo_basis ki kj =
 
 
 let is_internal det_space =
+    let mo_class      = DeterminantSpace.mo_class det_space in
+    let mo_num = Array.length @@ MOClass.mo_class_array mo_class in
     let m l =
       List.fold_left (fun accu i ->
-          let j = i-1 in Bitstring.(logor accu (shift_left_one j))
-        ) Bitstring.zero l
+          let j = i-1 in Bitstring.logor accu (Bitstring.shift_left_one mo_num j)
+        ) (Bitstring.zero mo_num) l
     in
-    let mo_class   = DeterminantSpace.mo_class det_space in
-    let aux_mask   = m (MOClass.auxiliary_mos   mo_class) in
+    let aux_mask   = m (MOClass.auxiliary_mos mo_class) in
     fun a ->
       let alfa =
         Determinant.alfa a
         |> Spindeterminant.bitstring
       in
+      (*
+      let beta =
+        Determinant.beta a
+        |> Spindeterminant.bitstring
+      in
+      let a = Bitstring.logand aux_mask alfa
+      and b = Bitstring.logand aux_mask beta
+      in Bitstring.popcount a + Bitstring.popcount b < 2
+      *)
+
       if not (Bitstring.logand aux_mask alfa |> Bitstring.is_zero ) then 
         false
       else
diff --git a/CI/Spindeterminant.ml b/CI/Spindeterminant.ml
index 241ae0b..1f6aff2 100644
--- a/CI/Spindeterminant.ml
+++ b/CI/Spindeterminant.ml
@@ -23,8 +23,8 @@ let bitstring = function
   | None -> invalid_arg "Spindeterminant is None"
 
 
-let vac = 
-  Some  { bitstring = Bitstring.zero;
+let vac n = 
+  Some  { bitstring = Bitstring.zero n;
           phase     = Phase.Pos; }
 
 
@@ -36,7 +36,8 @@ let creation p = function
       None
     else
       begin
-        let x = Bitstring.shift_left_one i in
+        let numbits = Bitstring.numbits spindet.bitstring in
+        let x = Bitstring.shift_left_one numbits i in
         let bitstring = Bitstring.logor spindet.bitstring x in
         let mask = Bitstring.minus_one x in
         let r = Bitstring.logand bitstring mask in
@@ -53,7 +54,8 @@ let annihilation h = function
       None
     else
       begin
-        let x = Bitstring.shift_left_one i in
+        let numbits = Bitstring.numbits spindet.bitstring in
+        let x = Bitstring.shift_left_one numbits i in
         let mask = Bitstring.minus_one x in
         let r = Bitstring.logand spindet.bitstring mask in
         let phase = Phase.add_nperm spindet.phase (Bitstring.popcount r) in
@@ -81,16 +83,16 @@ let degree t =
 
 let holes_of t t' =
   Bitstring.logand (bitstring t) (Bitstring.logxor (bitstring t) (bitstring t'))
-  |> Bitstring.to_list []
+  |> Bitstring.to_list 
 
 let particles_of t t' =
   Bitstring.logand (bitstring t') (Bitstring.logxor (bitstring t) (bitstring t'))
-  |> Bitstring.to_list []
+  |> Bitstring.to_list 
 
 let holes_particles_of t t' =
   let x = Bitstring.logxor (bitstring t) (bitstring t') in
-  let holes = Bitstring.logand (bitstring t) x |> Bitstring.to_list []
-  and particles = Bitstring.logand (bitstring t') x |> Bitstring.to_list []
+  let holes = Bitstring.logand (bitstring t) x |> Bitstring.to_list 
+  and particles = Bitstring.logand (bitstring t') x |> Bitstring.to_list 
   in
   List.map2 (fun h p -> (h,p)) holes particles
 
@@ -103,9 +105,9 @@ let negate_phase = function
 let of_bitstring ?(phase=Phase.Pos) bitstring =
   Some { bitstring ; phase }
 
-let of_list l = 
+let of_list n l = 
   List.rev l
-  |> List.fold_left (fun accu p -> creation p accu) vac
+  |> List.fold_left (fun accu p -> creation p accu) (vac n)
 
 
 let rec to_list = function
@@ -127,7 +129,7 @@ let n_electrons = function
 let pp_spindet n ppf = function
   | None -> Format.fprintf ppf "@[<h>None@]"
   | Some s -> 
-      Format.fprintf ppf "@[<h>%a %a@]" Phase.pp_phase s.phase (Bitstring.pp_bitstring n)
+      Format.fprintf ppf "@[<h>%a %a@]" Phase.pp_phase s.phase Bitstring.pp
       s.bitstring
 
 
@@ -140,36 +142,36 @@ let test_case () =
 
   let test_creation () =
     let l_a = [ 1 ; 2 ; 3 ; 5 ] in
-    let det = of_list l_a in
+    let det = of_list 10 l_a in
     Alcotest.(check (list int )) "bitstring 1" l_a (to_list det);
     Alcotest.(check bool) "phase 2" true (phase det = Phase.Pos);
     let l_b = [ 1 ; 3 ; 2 ; 5 ] in
-    let det = of_list l_b in
+    let det = of_list 10 l_b in
     Alcotest.(check (list int )) "bitstring 2" l_a (to_list det);
     Alcotest.(check bool) "phase 2" true (phase det = Phase.Neg);
   in
 
   let test_a_operators () =
     let det = 
-      creation 5 @@ creation 2 @@ creation 2 @@ creation 1 @@ vac
+      creation 5 @@ creation 2 @@ creation 2 @@ creation 1 @@ (vac 10)
     in
     Alcotest.(check bool) "none 1" true (is_none det);
 
     let det = 
-      creation 5 @@ creation 3 @@ creation 2 @@ creation 1 @@ vac
+      creation 5 @@ creation 3 @@ creation 2 @@ creation 1 @@ (vac 10)
     in
     let l_a = [ 1 ; 2 ; 3 ; 5 ] in
     Alcotest.(check (list int )) "bitstring 1" l_a (to_list det);
     Alcotest.(check bool) "phase 1" true (phase det = Phase.Pos);
 
     let det = 
-      creation 1 @@ creation 3 @@ creation 2 @@ creation 5 @@ vac
+      creation 1 @@ creation 3 @@ creation 2 @@ creation 5 @@ (vac 10)
     in
     Alcotest.(check (list int )) "bitstring 2" l_a (to_list det);
     Alcotest.(check bool) "phase 2" true (phase det = Phase.Neg);
 
     let l_b = [ 1 ; 3 ; 2 ; 5 ] in
-    let det = of_list l_b in
+    let det = of_list 10 l_b in
     Alcotest.(check (list int )) "bitstring 3" l_a (to_list det);
     Alcotest.(check bool) "phase 3" true (phase det = Phase.Neg);
 
@@ -197,16 +199,19 @@ let test_case () =
 
   let test_exc_operators () =
     let l_a = [ 1 ; 2 ; 3 ; 5 ] in
-    let det = of_list l_a in
+    let det = of_list 10 l_a in
     let l_b = [ 1 ; 7 ; 3 ; 5 ] in
-    let det2 = of_list l_b in
+    let det2 = of_list 10 l_b in
+    Format.printf "%a@." (pp_spindet 7) det;
+    Format.printf "%a@." (pp_spindet 7) det2;
+    Format.printf "%a@." (pp_spindet 7) (single_excitation_reference 2 7 det);
     Alcotest.(check bool) "single 1" true (single_excitation_reference 2 7 det = det2);
     Alcotest.(check bool) "single 2" true (single_excitation 2 7 det = single_excitation_reference 2 7 det);
     Alcotest.(check bool) "single 3" true (single_excitation_reference 4 7 det |> is_none);
     Alcotest.(check bool) "single 4" true (single_excitation 4 7 det |> is_none);
 
     let l_c = [ 1 ; 7 ; 6 ; 5 ] in
-    let det3 = of_list l_c in
+    let det3 = of_list 10 l_c in
     Alcotest.(check bool) "double 1" true (double_excitation_reference 2 7 3 6 det = det3);
     Alcotest.(check bool) "double 2" true (double_excitation 2 7 3 6 det = double_excitation_reference 2 7 3 6 det);
     Alcotest.(check bool) "double 3" true (double_excitation_reference 4 7 3 6 det |> is_none);
@@ -215,14 +220,14 @@ let test_case () =
 
   let test_exc_spindet () =
     let l_a = [ 1 ; 2 ; 3 ; 5 ] in
-    let det = of_list l_a in
+    let det = of_list 10 l_a in
     let l_b = [ 1 ; 7 ; 3 ; 5 ] in
-    let det2 = of_list l_b in
+    let det2 = of_list 10 l_b in
     Alcotest.(check int) "single" 1 (degree det det2);
     Alcotest.(check (list int)) "holes" [2] (holes_of det det2);
     Alcotest.(check (list int)) "particles" [7] (particles_of det det2);
     let l_b = [ 1 ; 7 ; 3 ; 6 ] in
-    let det2 = of_list l_b in
+    let det2 = of_list 10 l_b in
     Alcotest.(check int) "double" 2 (degree det det2);
     Alcotest.(check (list int)) "holes" [2 ; 5] (holes_of det det2);
     Alcotest.(check (list int)) "particles" [6 ; 7] (particles_of det det2);
diff --git a/CI/Spindeterminant.mli b/CI/Spindeterminant.mli
index e9df5a1..d87df4f 100644
--- a/CI/Spindeterminant.mli
+++ b/CI/Spindeterminant.mli
@@ -29,8 +29,9 @@ val negate_phase : t -> t
 
 (** {1 Second quantization operators} *)
 
-val vac : t
-(** Vacuum state, [vac = Some ]{% $|\rangle$ %}  *)
+val vac : int -> t
+(** Vacuum state, [vac = Some ]{% $|\rangle$ %}. The integer parameter contains the
+   number of orbitals in the basis set. *)
 
 val creation : particle -> t -> t
 (** [creation p] is the creation operator {% $a^\dagger_p$ %}. *)
@@ -66,10 +67,11 @@ val n_electrons : t -> int
 val of_bitstring : ?phase:Phase.t -> Bitstring.t -> t
 (** Creates from a bitstring and an optional phase.*)
 
-val of_list : int list -> t
+val of_list : int -> int list -> t
 (** Builds a spin-determinant from a list of orbital indices. If the creation of the
     spin-determinant is not possible because of Pauli's exclusion principle, a [None]
-    spin-determinant is returned. *)
+    spin-determinant is returned. 
+    The first integer is the size of the MO basis set. *)
 
 val to_list : t -> int list
 (** Transforms a spin-determinant into a list of orbital indices. *)
diff --git a/CI/SpindeterminantSpace.ml b/CI/SpindeterminantSpace.ml
index d5da05c..589687b 100644
--- a/CI/SpindeterminantSpace.ml
+++ b/CI/SpindeterminantSpace.ml
@@ -17,8 +17,9 @@ let fci_of_mo_basis ~frozen_core mo_basis elec_num =
   let mo_num = MOBasis.size mo_basis in
   let mo_class = MOClass.fci ~frozen_core mo_basis in
   let m l = 
-    List.fold_left (fun accu i -> let j = i-1 in Bitstring.(logor accu (shift_left_one j)) 
-                   ) Bitstring.zero l
+    List.fold_left (fun accu i -> let j = i-1 in
+      Bitstring.logor accu (Bitstring.shift_left_one mo_num j)
+                   ) (Bitstring.zero mo_num) l
   in
   let occ_mask    = m (MOClass.core_mos    mo_class)
   and active_mask = m (MOClass.active_mos  mo_class)
@@ -39,8 +40,9 @@ let cas_of_mo_basis mo_basis ~frozen_core elec_num n m =
   let mo_num = MOBasis.size mo_basis in
   let mo_class = MOClass.cas_sd ~frozen_core mo_basis n m in
   let m l = 
-    List.fold_left (fun accu i -> let j = i-1 in Bitstring.(logor accu (Bitstring.shift_left_one j)) 
-                   ) Bitstring.zero l
+    List.fold_left (fun accu i -> let j = i-1 in
+      Bitstring.logor accu (Bitstring.shift_left_one mo_num j)
+                   ) (Bitstring.zero mo_num) l
   in
   let active_mask   = m (MOClass.active_mos   mo_class) in
   let occ_mask      = m (MOClass.core_mos     mo_class) in
diff --git a/Utils/Bitstring.ml b/Utils/Bitstring.ml
index 46fe8e6..3e6382c 100644
--- a/Utils/Bitstring.ml
+++ b/Utils/Bitstring.ml
@@ -1,131 +1,171 @@
+module One = struct
+  type t = int
+
+
+  let of_int x =
+    assert (x > 0); x
+
+  let numbits _ = 63
+  let zero = 0
+  let is_zero x = x = 0
+  let shift_left x i = x lsl i
+  let shift_right x i = x lsr i
+  let shift_left_one i = 1 lsl i
+  let testbit x i = ( (x lsr i) land 1 ) = 1  
+  let logor a b = a lor b
+  let logxor a b = a lxor b
+  let logand a b = a land b
+  let lognot a = lnot a
+  let minus_one a = a - 1
+  let plus_one a = a + 1
+
+  let popcount = function
+    | 0 -> 0
+    | r  -> Util.popcnt (Int64.of_int r) 
+
+
+  let trailing_zeros r = 
+    Util.trailz (Int64.of_int r) 
+
+
+  let hamdist a b =
+    a lxor b
+    |> Int64.of_int
+    |> Util.popcnt 
+
+
+  let pp ppf s = 
+    Format.fprintf ppf "@[@[%a@]i@]" (Util.pp_bitstring 64) 
+      (Z.of_int s)
+
+end
+
+
+
+
+module Many = struct
+
+  type t = Z.t
+
+  let of_int = Z.of_int
+  let of_z x = x
+  let zero = Z.zero
+  let is_zero x = x = Z.zero
+  let shift_left = Z.shift_left 
+  let shift_right = Z.shift_right
+  let shift_left_one i = Z.shift_left Z.one i
+  let testbit = Z.testbit
+  let logor = Z.logor
+  let logxor = Z.logxor
+  let logand = Z.logand
+  let lognot = Z.lognot
+  let minus_one = Z.pred
+  let plus_one = Z.succ
+  let trailing_zeros = Z.trailing_zeros
+  let hamdist = Z.hamdist
+  let numbits i = max (Z.numbits i) 64
+
+  let popcount z = 
+    if z = Z.zero then 0 else Z.popcount z
+
+  let pp ppf s = 
+    Format.fprintf ppf "@[@[%a@]m@]" (Util.pp_bitstring (Z.numbits s))  s
+
+end
+
+
 type t = 
-| One  of int
+| One of int
 | Many of Z.t
 
+let of_int x =
+  One (One.of_int x)
 
-(*
-let of_int x = One x
-*)
-let of_int x = Many (Z.of_int x)
+let of_z x =
+  if Z.numbits x < 64 then One (Z.to_int x) else Many (Many.of_z x)
 
-let of_z x = if (Z.lt x (Z.of_int max_int)) then One (Z.to_int x) else Many x
-
-(*
-let zero = One 0
-*)
-let zero = Many Z.zero
+let zero = function
+| n when n < 64 -> One (One.zero)
+| _ -> Many (Many.zero)
 
+let numbits = function
+| One x -> One.numbits x
+| Many x -> Many.numbits x
 
 let is_zero = function
-| One  x -> assert false (* x = 0 *)
-| Many x -> x = Z.zero
+| One x -> One.is_zero x
+| Many x -> Many.is_zero x
 
+let shift_left x i = match x with
+| One x -> One (One.shift_left x i)
+| Many x -> Many (Many.shift_left x i)
 
-let shift_left x i = 
-  match x with
-  | One x -> assert false (*
-    let y = x lsl i in
-    if y lsr i = x then
-      One y
-    else
-      Many (Z.shift_left (Z.of_int x) i)
-      *)
-  | Many x -> Many (Z.shift_left x i)
+let shift_right x i = match x with
+| One x -> One (One.shift_right x i)
+| Many x -> Many (Many.shift_right x i)
 
+let shift_left_one = function
+| n when n < 64 -> fun i -> One (One.shift_left_one i)
+| _ -> fun i -> Many (Many.shift_left_one i)
 
-let shift_right x i = 
-  match x with
-  | One x -> assert false (* One (x lsr i)  *)
-  | Many x -> Z.shift_right x i |> of_z
-
-
-let shift_left_one =
-  let memo =
-    Array.init 512 (fun i ->
-    (*
-      if i < 63 then
-        One (1 lsl i)
-      else
-        *)
-        Many (Z.(shift_left one i)))
-  in
-  fun i ->
-    if i < 512 then
-      memo.(i)
-    else
-      Many (Z.(shift_left one i))
-
-
-let testbit bs i =
-  match bs with
-  | One one -> assert false (* (one lsr i) land 1 = 1  *)
-  | Many z -> Z.testbit z i
-
+let testbit = function
+| One x -> One.testbit x
+| Many x -> Many.testbit x
 
 let logor a b = 
   match a,b with
-  | One  a, One  b -> assert false (* One  (a lor b) *)
-  | One  a, Many b -> Many (Z.logor (Z.of_int a) b)
-  | Many a, One  b -> Many (Z.logor a (Z.of_int b))
-  | Many a, Many b -> Many (Z.logor a b)
-
+  | One a, One b -> One (One.logor a b)
+  | Many a, Many b -> Many (Many.logor a b)
+  | _ -> invalid_arg "Bitstring.logor"
 
 let logxor a b = 
   match a,b with
-  | One  a, One  b -> assert false (* One  (a lxor b) *)
-  | One  a, Many b -> Many (Z.logxor (Z.of_int a) b)
-  | Many a, One  b -> Many (Z.logxor a (Z.of_int b))
-  | Many a, Many b -> Many (Z.logxor a b)
-
+  | One a, One b -> One (One.logxor a b)
+  | Many a, Many b -> Many (Many.logxor a b)
+  | _ -> invalid_arg "Bitstring.logxor"
 
 let logand a b = 
   match a,b with
-  | One  a, One  b -> assert false (* One  (a land b) *)
-  | One  a, Many b -> Many (Z.logand (Z.of_int a) b)
-  | Many a, One  b -> Many (Z.logand a (Z.of_int b))
-  | Many a, Many b -> Many (Z.logand a b)
-
+  | One a, One b -> One (One.logand a b)
+  | Many a, Many b -> Many (Many.logand a b)
+  | _ -> invalid_arg "Bitstring.logand"
 
 let lognot = function
-  | One  a -> Many ( Z.(lognot @@ of_int a) )
-  | Many a -> Many ( Z.lognot a )
-
+| One x -> One (One.lognot x)
+| Many x -> Many (Many.lognot x)
 
 let minus_one = function
-  | One  a -> assert false (* One  ( a-1 ) *)
-  | Many a -> Many ( Z.(a-one) )
-
+| One x -> One (One.minus_one x)
+| Many x -> Many (Many.minus_one x)
 
 let plus_one = function
-  | One  a -> assert false (* One  ( a+1 ) *)
-  | Many a -> Many ( Z.(a+one) )
-
-
-let popcount = function
-  | One r  -> assert false (* Util.popcnt (Int64.of_int r) *)
-  | Many r when r = Z.zero -> 0
-  | Many r -> Z.popcount r
-
+| One x -> One (One.plus_one x)
+| Many x -> Many (Many.plus_one x)
 
 let trailing_zeros = function
-  | One r  -> assert false (* Util.trailz (Int64.of_int r) *)
-  | Many r -> Z.trailing_zeros r
+| One x -> One.trailing_zeros x
+| Many x -> Many.trailing_zeros x
+
+let hamdist a b = match a, b with
+| One a, One b -> One.hamdist a b
+| Many a, Many b -> Many.hamdist a b
+| _ -> invalid_arg "Bitstring.hamdist"
+
+let popcount = function
+| One x -> One.popcount x
+| Many x -> Many.popcount x
+
+let pp ppf = function
+| One x -> One.pp ppf x
+| Many x -> Many.pp ppf x
 
 
-let hamdist a b =
-  match a,b with
-  | One  a, One  b -> assert false (* a lxor b |> Int64.of_int |> Util.popcnt *)
-  | One  a, Many b -> Z.hamdist (Z.of_int a) b
-  | Many a, One  b -> Z.hamdist a (Z.of_int b)
-  | Many a, Many b -> Z.hamdist a b
-
-
-let rec to_list accu = function
-  | t when (t = Many Z.zero || t = One 0) -> List.rev accu
+let rec to_list ?(accu=[]) = function
+  | t when (is_zero t) -> List.rev accu
   | t -> let newlist =
            (trailing_zeros t + 1)::accu
          in
-         to_list newlist (logand t (minus_one t))
+         to_list ~accu:newlist (logand t (minus_one t))
 
 
 (** [permtutations m n] generates the list of all possible [n]-bit
@@ -149,118 +189,46 @@ let permtutations m n =
       let t'' = shift_right (minus_one (logand (lognot t) t')) (trailing_zeros u + 1) in
       aux (k-1) (logor t' t'') (u :: rest)
   in
-  aux (Util.binom n m) (minus_one (shift_left_one m)) []
-
-
-
-let pp_bitstring n ppf s = 
-  Format.fprintf ppf "@[%a@]" (Util.pp_bitstring n) (
-  match s with
-  | Many b -> b
-  | One b -> Z.of_int b )
-
-
+  aux (Util.binom n m) (minus_one (shift_left_one m m)) []
 
 
 (*-----------------------------------------------------------------------------------*)
-(* TODO 
 
 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 1" l_a (to_list det);
-    Alcotest.(check bool) "phase 2" true (phase det = Phase.Pos);
-    let l_b = [ 1 ; 3 ; 2 ; 5 ] in
-    let det = of_list l_b in
-    Alcotest.(check (list int )) "bitstring 2" l_a (to_list det);
-    Alcotest.(check bool) "phase 2" true (phase det = Phase.Neg);
+  let test_one_many () =
+    let x = 8745687 in
+    let z = Z.of_int 8745687 in
+    let one_x  = One x in
+    let many_x = Many z in
+    Alcotest.(check bool) "of_x" true (one_x = (of_int x));
+    Alcotest.(check bool) "of_z" true (one_x = (of_z z));
+    Alcotest.(check bool) "shift_left1" true (One (x lsl 3) = shift_left one_x 3);
+    Alcotest.(check bool) "shift_left2" true (Many (Z.shift_left z 3) = shift_left many_x 3);
+    Alcotest.(check bool) "shift_left3" true (Many (Z.shift_left z 100) = shift_left many_x 100);
+    Alcotest.(check bool) "shift_right1" true (One (x lsr 3) = shift_right one_x 3);
+    Alcotest.(check bool) "shift_right2" true (Many (Z.shift_right z 3) = shift_right many_x 3);
+    Alcotest.(check bool) "shift_left_one1" true (One (1 lsl 3) = shift_left_one 4 3);
+    Alcotest.(check bool) "shift_left_one2" true (Many (Z.shift_left Z.one 200) = shift_left_one 300 200);
+    Alcotest.(check bool) "testbit1" true  (testbit (One 8) 3);
+    Alcotest.(check bool) "testbit2" false (testbit (One 8) 2);
+    Alcotest.(check bool) "testbit3" false (testbit (One 8) 4);
+    Alcotest.(check bool) "testbit4" true  (testbit (Many (Z.of_int 8)) 3);
+    Alcotest.(check bool) "testbit5" false (testbit (Many (Z.of_int 8)) 2);
+    Alcotest.(check bool) "testbit6" false (testbit (Many (Z.of_int 8)) 4);
+    Alcotest.(check bool) "logor1" true (One (1 lor 2) = logor (One 1) (One 2));
+    Alcotest.(check bool) "logor2" true (Many (Z.of_int (1 lor 2)) = logor (Many Z.one) (Many (Z.of_int 2)));
+    Alcotest.(check bool) "logxor1" true (One (1 lxor 2) = logxor (One 1) (One 2));
+    Alcotest.(check bool) "logxor2" true (Many (Z.of_int (1 lxor 2)) = logxor (Many Z.one) (Many (Z.of_int 2)));
+    Alcotest.(check bool) "logand1" true (One (1 land 2) = logand (One 1) (One 2));
+    Alcotest.(check bool) "logand2" true (Many (Z.of_int (1 land 2)) = logand (Many Z.one) (Many (Z.of_int 2)));
+    Alcotest.(check bool) "to_list" true ([ 1 ; 3 ; 4 ; 6 ] = (to_list (One 45)));
   in
 
-  let test_a_operators () =
-    let det = 
-      creation 5 @@ creation 2 @@ creation 2 @@ creation 1 @@ vac
-    in
-    Alcotest.(check bool) "none 1" true (is_none det);
-
-    let det = 
-      creation 5 @@ creation 3 @@ creation 2 @@ creation 1 @@ vac
-    in
-    let l_a = [ 1 ; 2 ; 3 ; 5 ] in
-    Alcotest.(check (list int )) "bitstring 1" l_a (to_list det);
-    Alcotest.(check bool) "phase 1" true (phase det = Phase.Pos);
-
-    let det = 
-      creation 1 @@ creation 3 @@ creation 2 @@ creation 5 @@ vac
-    in
-    Alcotest.(check (list int )) "bitstring 2" l_a (to_list det);
-    Alcotest.(check bool) "phase 2" true (phase det = Phase.Neg);
-
-    let l_b = [ 1 ; 3 ; 2 ; 5 ] in
-    let det = of_list l_b in
-    Alcotest.(check (list int )) "bitstring 3" l_a (to_list det);
-    Alcotest.(check bool) "phase 3" true (phase det = Phase.Neg);
-
-    Alcotest.(check bool) "none 1" true (annihilation 4 det |> is_none);
-
-    let det = 
-      annihilation 1 det
-    in
-    Alcotest.(check (list int )) "bitstring 4" (List.tl l_a) (to_list det);
-    Alcotest.(check bool) "phase 4" true (phase det = Phase.Neg);
-
-    let det = 
-      annihilation 3 det 
-    in
-    Alcotest.(check (list int )) "bitstring 5" [ 2 ; 5 ] (to_list det);
-    Alcotest.(check bool) "phase 5" true (phase det = Phase.Pos);
-
-    let det = 
-      annihilation 5 @@ annihilation 2 det
-    in
-    Alcotest.(check (list int )) "bitstring 6" [] (to_list det);
-    Alcotest.(check bool) "phase 6" true (phase det = Phase.Pos);
-
-  in
-
-  let test_exc_operators () =
-    let l_a = [ 1 ; 2 ; 3 ; 5 ] in
-    let det = of_list l_a in
-    let l_b = [ 1 ; 7 ; 3 ; 5 ] in
-    let det2 = of_list l_b in
-    Alcotest.(check bool) "single 1" true (single_excitation_reference 2 7 det = det2);
-    Alcotest.(check bool) "single 2" true (single_excitation 2 7 det = single_excitation_reference 2 7 det);
-    Alcotest.(check bool) "single 3" true (single_excitation_reference 4 7 det |> is_none);
-    Alcotest.(check bool) "single 4" true (single_excitation 4 7 det |> is_none);
-
-    let l_c = [ 1 ; 7 ; 6 ; 5 ] in
-    let det3 = of_list l_c in
-    Alcotest.(check bool) "double 1" true (double_excitation_reference 2 7 3 6 det = det3);
-    Alcotest.(check bool) "double 2" true (double_excitation 2 7 3 6 det = double_excitation_reference 2 7 3 6 det);
-    Alcotest.(check bool) "double 3" true (double_excitation_reference 4 7 3 6 det |> is_none);
-    Alcotest.(check bool) "double 4" true (double_excitation 4 7 3 6 det |> is_none);
-  in
-
-  let test_exc_spindet () =
-    let l_a = [ 1 ; 2 ; 3 ; 5 ] in
-    let det = of_list l_a in
-    let l_b = [ 1 ; 7 ; 3 ; 5 ] in
-    let det2 = of_list l_b in
-    Alcotest.(check int) "single" 1 (degree det det2);
-    Alcotest.(check (list int)) "holes" [2] (holes_of det det2);
-    Alcotest.(check (list int)) "particles" [7] (particles_of det det2);
-    let l_b = [ 1 ; 7 ; 3 ; 6 ] in
-    let det2 = of_list l_b in
-    Alcotest.(check int) "double" 2 (degree det det2);
-    Alcotest.(check (list int)) "holes" [2 ; 5] (holes_of det det2);
-    Alcotest.(check (list int)) "particles" [6 ; 7] (particles_of det det2);
-  in
   [
-    "Creation", `Quick, test_creation;
-    "Creation/Annihilation Operators", `Quick, test_a_operators;
-    "Excitation Operators", `Quick, test_exc_operators;
-    "Excitation of spindet", `Quick, test_exc_spindet;
+    "One-many", `Quick, test_one_many;
   ]
 
-*)
+
+
+
diff --git a/Utils/Util.mli b/Utils/Util.mli
index c716ec4..53212d7 100644
--- a/Utils/Util.mli
+++ b/Utils/Util.mli
@@ -32,6 +32,9 @@ val popcnt : int64 -> int
 val trailz : int64 -> int
 (** ctz instruction *)
 
+val leadz : int64 -> int
+(** ctz instruction *)
+
 
 
 (** {2 General functions} *)
diff --git a/run_tests.ml b/run_tests.ml
index 9e66476..5bdeebb 100644
--- a/run_tests.ml
+++ b/run_tests.ml
@@ -14,6 +14,7 @@ let test_water_dz () =
   in
   Alcotest.run "Unit tests" [
     "Util", Util.test_case ();
+    "Bitstring", Bitstring.test_case ();
     "Spindeterminant", Spindeterminant.test_case ();
     "Determinant", Determinant.test_case ();
     "Excitation", Excitation.test_case ();