diff --git a/Utils/FourIdxStorage.ml b/Utils/FourIdxStorage.ml
index cea66ad..191f702 100644
--- a/Utils/FourIdxStorage.ml
+++ b/Utils/FourIdxStorage.ml
@@ -44,6 +44,13 @@ let dense_index i j size =
   (j-1)*size + i
 
 
+let sym_index i j =
+  if i < j then
+    (j*(j-1))/2 + i
+  else
+    (i*(i-1))/2 + j
+
+
 let unsafe_get_four_index ~r1 ~r2 t = 
 
   let open Bigarray.Array2 in
@@ -78,7 +85,7 @@ let unsafe_get_four_index ~r1 ~r2 t =
 
   else
     match t.four_index with
-    | Dense  a -> unsafe_get a (dense_index i j t.size) (dense_index k l t.size)
+    | Dense  a -> unsafe_get a (dense_index i k t.size) (sym_index j l)
     | Sparse a -> let key = key_of_indices ~r1 ~r2 in
                   try Hashtbl.find a key
                   with Not_found -> 0.
@@ -152,24 +159,18 @@ let unsafe_set_four_index ~r1 ~r2 ~value t =
   else
 
     match t.four_index with
-    | Dense  a -> let ij = (dense_index i j t.size)
-                  and kl = (dense_index k l t.size)
-                  and il = (dense_index i l t.size)
-                  and kj = (dense_index k j t.size)
-                  and ji = (dense_index j i t.size)
-                  and lk = (dense_index l k t.size)
-                  and li = (dense_index l i t.size)
-                  and jk = (dense_index j k t.size)
+    | Dense  a -> let ik = (dense_index i k t.size)
+                  and jl = (dense_index j l t.size)
+                  and ki = (dense_index k i t.size)
+                  and lj = (dense_index l j t.size)
+                  and ik_s = (sym_index i k)
+                  and jl_s = (sym_index j l)
                   in
                   begin
-                    unsafe_set a ij kl value;
-                    unsafe_set a kj il value;
-                    unsafe_set a il kj value;
-                    unsafe_set a kl ij value;
-                    unsafe_set a ji lk value;
-                    unsafe_set a li jk value;
-                    unsafe_set a jk li value;
-                    unsafe_set a lk ji value
+                    unsafe_set a ik jl_s value;
+                    unsafe_set a ki jl_s value;
+                    unsafe_set a jl ik_s value;
+                    unsafe_set a lj ik_s value;
                   end
     | Sparse a -> let key = key_of_indices ~r1 ~r2 in
                   Hashtbl.replace a key value
@@ -233,7 +234,7 @@ let create ~size ?(temp_dir="/dev/shm") sparsity =
   let four_index = 
     match sparsity with
     | `Dense  -> let result =
-                     SharedMemory.create ~temp_dir Float64 [| size*size ; size*size |]
+                     SharedMemory.create ~temp_dir Float64 [| size*size ; (size*(size+1))/2 |]
                      |> Bigarray.array2_of_genarray
                  in
                  Dense result