diff --git a/CI/Spindeterminant.ml b/CI/Spindeterminant.ml
index dccd981..c12d774 100644
--- a/CI/Spindeterminant.ml
+++ b/CI/Spindeterminant.ml
@@ -5,6 +5,8 @@ type s =
   }
 
 type t = s option
+type hole = int
+type particle = int
 
 let phase = function 
   | Some s -> s.phase
@@ -42,10 +44,10 @@ let creation p = function
       end
 
 
-let annihilation q = function
+let annihilation h = function
   | None -> None
   | Some spindet ->
-    let i = pred q in
+    let i = pred h in
     if not (Z.testbit spindet.bitstring i) then
       None
     else
@@ -58,7 +60,11 @@ let annihilation q = function
         Some { bitstring ; phase }
       end
 
+let single_excitation_reference h p spindet = 
+      creation p @@ annihilation h @@ spindet
 
+let single_excitation p q = 
+  single_excitation_reference p q
 
 let of_list l = 
   List.rev l
@@ -105,7 +111,7 @@ let test_case () =
     Alcotest.(check bool) "phase 2" true (phase det = Phase.Neg);
   in
 
-  let test_operators () =
+  let test_a_operators () =
     let det = 
       creation 5 @@ creation 2 @@ creation 2 @@ creation 1 @@ vac
     in
@@ -150,9 +156,19 @@ let test_case () =
     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 2 7 det = det2);
+    Alcotest.(check bool) "single 1" true (single_excitation 4 7 det |> is_none);
+  in
   [
     "Creation", `Quick, test_creation;
-    "Operators", `Quick, test_operators;
+    "Creation/Annihilation Operators", `Quick, test_a_operators;
+    "Excitation Operators", `Quick, test_exc_operators;
   ]
 
 
diff --git a/CI/Spindeterminant.mli b/CI/Spindeterminant.mli
index b266e06..de49a5f 100644
--- a/CI/Spindeterminant.mli
+++ b/CI/Spindeterminant.mli
@@ -5,6 +5,8 @@ as a bit string and a phase factor.
 *)
 
 type t 
+type hole = int
+type particle = int
 
 (** {1 Accessors}. *)
 
@@ -27,11 +29,15 @@ val is_none : t -> bool
 val vac : t
 (** Vacuum state, [vac = Some ]{% $|\rangle$ %}  *)
 
-val creation : int -> t -> t
+val creation : particle -> t -> t
 (** [creation p] is the creation operator {% $a^\dagger_p$ %}. *)
 
-val annihilation : int -> t -> t
-(** [annihilation q] is the annihilation operator {% $a_q$ %}. *)
+val annihilation : hole -> t -> t
+(** [annihilation h] is the annihilation operator {% $a_h$ %}. *)
+
+val single_excitation : hole -> particle -> t -> t
+(** Single excitation operator {% $T_h^p = a^\dagger_p a_h$ %}. *)
+
 
 val of_list : int list -> t
 (** Builds a spin-determinant from a list of orbital indices. If the creation of the