diff --git a/src/utils_trust_region/vec_to_mat_index.irp.f b/src/utils_trust_region/vec_to_mat_index.irp.f
new file mode 100644
index 00000000..5d68748b
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+++ b/src/utils_trust_region/vec_to_mat_index.irp.f
@@ -0,0 +1,71 @@
+! Vector to matrix indexes
+  
+! *Compute the indexes p,q of a matrix element with the vector index i*
+
+! Vector (i) -> lower diagonal matrix (p,q), p > q
+
+! If a matrix is antisymmetric it can be reshaped as a vector. And the
+! vector can be reshaped as an antisymmetric matrix
+
+! \begin{align*}
+! \begin{pmatrix}
+! 0 & -1 & -2 & -4 \\
+! 1 & 0  & -3 & -5 \\
+! 2 & 3 & 0  & -6  \\
+! 4 & 5 & 6 & 0
+! \end{pmatrix}
+! \Leftrightarrow
+! \begin{pmatrix}
+! 1 & 2 & 3 & 4 & 5 & 6
+! \end{pmatrix}
+! \end{align*}
+
+! !!! Here the algorithm only work for the lower diagonal !!!
+
+! Input:
+! | i | integer | index in the vector |
+
+! Ouput:
+! | p,q | integer | corresponding indexes in the lower diagonal of a matrix |
+! |     |         | p > q,                                                  |
+! |     |         | p -> row,                                               |
+! |     |         | q -> column                                             |
+
+
+subroutine vec_to_mat_index(i,p,q)
+
+  include 'pi.h'
+
+  BEGIN_DOC
+  ! Compute the indexes (p,q) of the element in the lower diagonal matrix knowing
+  ! its index i a vector
+  END_DOC
+
+  implicit none
+
+  ! Variables
+
+  ! in
+  integer,intent(in)   :: i
+  
+  ! out
+  integer, intent(out) :: p,q
+  
+  ! internal 
+  integer              :: a,b
+  double precision     :: da
+
+  da = 0.5d0*(1+ sqrt(1d0+8d0*DBLE(i)))
+  a = INT(da) 
+  if ((a*(a-1))/2==i) then
+    p = a-1
+  else
+    p = a
+  endif
+  b = p*(p-1)/2
+ 
+  ! Matrix element indexes
+  p = p + 1
+  q = i - b 
+
+end subroutine