diff --git a/src/PROPERTIES/properties_ci.irp.f b/src/PROPERTIES/properties_ci.irp.f
index 691a122..fa14434 100644
--- a/src/PROPERTIES/properties_ci.irp.f
+++ b/src/PROPERTIES/properties_ci.irp.f
@@ -51,3 +51,31 @@ BEGIN_PROVIDER [ double precision, ci_h_psidet, (size_ci_h_psidet) ]
 END_PROVIDER
 
 
+BEGIN_PROVIDER [ double precision, ci_overlap_matrix, (size_ci_overlap_matrix) ]
+  implicit none
+  BEGIN_DOC
+  ! < det(i) | det(j) >
+  ! < det(i) | det(j) >
+  !
+  ! Dimensions : det_num
+  END_DOC
+
+  integer                        :: i, j, k, l, m, n
+  double precision :: f
+
+  do k=1,det_num
+    i = det_coef_matrix_rows(k)
+    j = det_coef_matrix_columns(k)
+    f = det_alpha_value(i)*det_beta_value (j)*psidet_inv*psidet_inv
+    do l=1,det_num
+      m = det_coef_matrix_rows(l)
+      n = det_coef_matrix_columns(l)
+      ci_overlap_matrix(l) = det_alpha_value(m)*det_beta_value(n) * f
+    enddo
+  enddo
+
+  ci_overlap_matrix_min = min(ci_overlap_matrix_min,minval(ci_overlap_matrix))
+  ci_overlap_matrix_max = max(ci_overlap_matrix_max,maxval(ci_overlap_matrix))
+  SOFT_TOUCH ci_overlap_matrix_min ci_overlap_matrix_max
+END_PROVIDER
+