diff --git a/.merlin b/.merlin
index 0b4411f..548e65e 100644
--- a/.merlin
+++ b/.merlin
@@ -7,4 +7,5 @@ S Utils
 S Basis
 S SCF
 S MOBasis
+S CI
 B _build/**
diff --git a/CI/Determinant.ml b/CI/Determinant.ml
new file mode 100644
index 0000000..c0fa584
--- /dev/null
+++ b/CI/Determinant.ml
@@ -0,0 +1,117 @@
+type sign_type =
+| Plus
+| Minus
+
+type spin_determinant_type = Z.t
+
+type determinant_type =
+{
+  alpha : spin_determinant_type ;
+  beta  : spin_determinant_type ;
+  sign  : sign_type;
+}
+
+
+type t = determinant_type
+
+
+let spindet_of_list l = 
+  let rec aux accu nperm = function
+  | [] -> 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 nperm =
+      let mask = Z.(x-one) in
+      let r = Z.(accu land 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 of_lists a b =
+  let alpha, sign_a = 
+    spindet_of_list a
+  in
+  let beta, sign_b = 
+    spindet_of_list b
+  in
+  let sign =
+    match sign_a, sign_b with
+    | Plus , Plus -> Plus
+    | Minus, Minus-> Plus
+    | _ -> Minus
+  in
+  { alpha ; beta ; sign }
+
+let alpha det = det.alpha
+let beta  det = det.beta
+
+let sgn det =
+  match det.sign with
+  | Plus  ->  1.
+  | Minus -> -1.
+
+let pp_spindet ppf spindet =
+  String.init (Z.numbits spindet) (fun i -> if (Z.testbit spindet i) then '+' else '-')
+  |> Format.fprintf ppf "@[<v> @[<h> %s @]@]"
+
+
+let pp_det ppf det =
+  Format.fprintf ppf "@[<v> a: %a @; b: %a @]@." pp_spindet det.alpha pp_spindet det.beta
+
+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 z_a = alpha det
+      and z_b = beta  det in
+      Alcotest.(check (list int )) "alpha" (to_list z_a) l_a;
+      Alcotest.(check (list int )) "beta"  (to_list z_b) l_b;
+      Alcotest.(check bool) "phase" (sgn det = 1.)  true;
+  in
+  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
+      Alcotest.(check bool) "phase" (sgn det = -1.)  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
+      Alcotest.(check bool) "phase" (sgn det = 1.)  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
+      Alcotest.(check bool) "phase" (sgn det = -1.)  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
+      Alcotest.(check bool) "phase" (sgn det = 1.)  true;
+  in
+  [
+    "Creation", `Quick, test_creation;
+    "Phase"   , `Quick, test_phase;
+  ]
+
+
+  
diff --git a/MOBasis/MOBasis.ml b/MOBasis/MOBasis.ml
index c3c3db9..2c57789 100644
--- a/MOBasis/MOBasis.ml
+++ b/MOBasis/MOBasis.ml
@@ -76,7 +76,7 @@ let four_index_transform ~mo_coef eri_ao =
           List.iter (fun k ->
               List.iter (fun j ->
                   incr jk;
-                  ERI.get_phys_all_i eri_ao ~j ~k ~l
+                  ERI.get_chem_all_i eri_ao ~j ~k ~l
                   |> Array.iteri (fun i x -> o.{i+1,!jk} <- x)
               ) range_ao
           ) range_ao;
@@ -116,10 +116,10 @@ let four_index_transform ~mo_coef eri_ao =
               List.iter (fun alpha ->
                  let x = u.{alpha,beta,gamma} in
                  if x <> 0. then
-                    ERI.set_phys eri_mo alpha beta gamma delta x
-              ) range_mo
+                    ERI.set_chem eri_mo alpha beta gamma delta x
+              ) (list_range ~start:1 beta) 
           ) range_mo
-      ) range_mo
+      ) (list_range ~start:1 delta) 
   ) range_mo;
   Printf.eprintf "\n%!";
 
diff --git a/Makefile.include b/Makefile.include
index 2c79cc4..54a0966 100644
--- a/Makefile.include
+++ b/Makefile.include
@@ -1,6 +1,6 @@
 .NOPARALLEL:
 
-INCLUDE_DIRS=Nuclei,Utils,Basis,SCF,MOBasis
+INCLUDE_DIRS=Nuclei,Utils,Basis,SCF,MOBasis,CI
 LIBS=
 PKGS=
 OCAMLBUILD=ocamlbuild -j 0 -cflags $(ocamlcflags) -lflags $(ocamlcflags) $(ocamldocflags) -Is $(INCLUDE_DIRS) -ocamlopt $(ocamloptflags)
diff --git a/_tags b/_tags
index 798b54d..c7598f6 100644
--- a/_tags
+++ b/_tags
@@ -1,4 +1,4 @@
-true: package(str,unix,bigarray,lacaml,alcotest)
+true: package(str,unix,bigarray,lacaml,alcotest,zarith)
 <*.byte>  : linkdep(Utils/math_functions.o), custom
 <*.native>: linkdep(Utils/math_functions.o)
 <odoc-ltxhtml>: not_hygienic
diff --git a/run_tests.ml b/run_tests.ml
index 123f6e3..6e66624 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;
+    "Determinant", Determinant.test_case ();
   ]