diff --git a/plugins/Hartree_Fock/HF_density_matrix_ao.irp.f b/plugins/Hartree_Fock/HF_density_matrix_ao.irp.f
index 597b88c7..e8585f59 100644
--- a/plugins/Hartree_Fock/HF_density_matrix_ao.irp.f
+++ b/plugins/Hartree_Fock/HF_density_matrix_ao.irp.f
@@ -4,7 +4,7 @@ BEGIN_PROVIDER [ double precision, HF_density_matrix_ao_alpha, (ao_num_align,ao_
    ! S^-1 x Alpha density matrix in the AO basis x S^-1
    END_DOC
    
-   call dgemm('N','T',ao_num,mo_tot_num,elec_alpha_num,1.d0, &
+   call dgemm('N','T',ao_num,ao_num,elec_alpha_num,1.d0, &
         mo_coef, size(mo_coef,1), &
         mo_coef, size(mo_coef,1), 0.d0, &
         HF_density_matrix_ao_alpha, size(HF_density_matrix_ao_alpha,1))
@@ -17,7 +17,7 @@ BEGIN_PROVIDER [ double precision, HF_density_matrix_ao_beta,  (ao_num_align,ao_
    ! S^-1 Beta density matrix in the AO basis x S^-1
    END_DOC
    
-   call dgemm('N','T',ao_num,mo_tot_num,elec_beta_num,1.d0, &
+   call dgemm('N','T',ao_num,ao_num,elec_beta_num,1.d0, &
         mo_coef, size(mo_coef,1), &
         mo_coef, size(mo_coef,1), 0.d0, &
         HF_density_matrix_ao_beta, size(HF_density_matrix_ao_beta,1))
diff --git a/plugins/Hartree_Fock/diagonalize_fock.irp.f b/plugins/Hartree_Fock/diagonalize_fock.irp.f
index 7f25851c..850ba0aa 100644
--- a/plugins/Hartree_Fock/diagonalize_fock.irp.f
+++ b/plugins/Hartree_Fock/diagonalize_fock.irp.f
@@ -10,58 +10,103 @@
    integer, allocatable           :: iwork(:)
    double precision, allocatable  :: work(:), F(:,:), S(:,:)
    
-   allocate(F(ao_num_align,ao_num), S(ao_num_align,ao_num) )
-   do j=1,ao_num
-     do i=1,ao_num
-       S(i,j) = ao_overlap(i,j)
-       F(i,j) = Fock_matrix_ao(i,j)
-     enddo
-   enddo
 
-   n = ao_num
-   lwork = 1+6*n + 2*n*n
-   liwork = 3 + 5*n
-   
-   allocate(work(lwork), iwork(liwork) )
+   if (mo_tot_num == ao_num) then
+       ! Solve H.C = E.S.C in AO basis set
 
-   lwork = -1
-   liwork = -1
+       allocate(F(ao_num_align,ao_num), S(ao_num_align,ao_num) )
+       do j=1,ao_num
+         do i=1,ao_num
+           S(i,j) = ao_overlap(i,j)
+           F(i,j) = Fock_matrix_ao(i,j)
+         enddo
+       enddo
 
-   call dsygvd(1,'v','u',ao_num,F,size(F,1),S,size(S,1),&
-     diagonal_Fock_matrix_mo, work, lwork, iwork, liwork, info)
-!    call dsygv(1, 'v', 'u',ao_num,F,size(F,1),S,size(S,1),&
-!     diagonal_Fock_matrix_mo, work, lwork, info)
+       n = ao_num
+       lwork = 1+6*n + 2*n*n
+       liwork = 3 + 5*n
+       
+       allocate(work(lwork), iwork(liwork) )
 
+       lwork = -1
+       liwork = -1
 
+       call dsygvd(1,'v','u',ao_num,F,size(F,1),S,size(S,1),&
+         diagonal_Fock_matrix_mo, work, lwork, iwork, liwork, info)
 
-   if (info /= 0) then
-     print *,  irp_here//' failed : ', info
-     stop 1
-   endif
-   lwork = int(work(1))
-   liwork = iwork(1)
-   deallocate(work,iwork)
-   allocate(work(lwork), iwork(liwork) )
-!   deallocate(work)
-!   allocate(work(lwork))
+       if (info /= 0) then
+         print *,  irp_here//' failed : ', info
+         stop 1
+       endif
+       lwork = int(work(1))
+       liwork = iwork(1)
+       deallocate(work,iwork)
+       allocate(work(lwork), iwork(liwork) )
 
-   call dsygvd(1,'v','u',ao_num,F,size(F,1),S,size(S,1),&
-     diagonal_Fock_matrix_mo, work, lwork, iwork, liwork, info)
+       call dsygvd(1,'v','u',ao_num,F,size(F,1),S,size(S,1),&
+         diagonal_Fock_matrix_mo, work, lwork, iwork, liwork, info)
 
-!    call dsygv(1, 'v', 'u',ao_num,F,size(F,1),S,size(S,1),&
-!     diagonal_Fock_matrix_mo, work, lwork, info)
+       if (info /= 0) then
+         print *,  irp_here//' failed : ', info
+         stop 1
+       endif
+       do j=1,mo_tot_num
+         do i=1,ao_num
+           eigenvectors_Fock_matrix_mo(i,j) = F(i,j)
+         enddo
+       enddo
 
-   if (info /= 0) then
-     print *,  irp_here//' failed : ', info
-     stop 1
-   endif
-   do j=1,mo_tot_num
-     do i=1,ao_num
-       eigenvectors_Fock_matrix_mo(i,j) = F(i,j)
-     enddo
-   enddo
+       deallocate(work, iwork, F, S)
+
+  else 
+
+       ! Solve H.C = E.C in MO basis set
+       
+       allocate( F(mo_tot_num_align,mo_tot_num) )
+       do j=1,mo_tot_num
+         do i=1,mo_tot_num
+           F(i,j) = Fock_matrix_mo(i,j)
+         enddo
+       enddo
+       
+       n = mo_tot_num
+       lwork = 1+6*n + 2*n*n
+       liwork = 3 + 5*n
+       
+       allocate(work(lwork), iwork(liwork) )
+       
+       lwork = -1
+       liwork = -1
+       
+       call dsyevd( 'V', 'U', mo_tot_num, F,                         &
+           size(F,1), diagonal_Fock_matrix_mo,                       &
+           work, lwork, iwork, liwork, info)
+       
+       if (info /= 0) then
+         print *,  irp_here//' failed : ', info
+         stop 1
+       endif
+       lwork = int(work(1))
+       liwork = iwork(1)
+       deallocate(work,iwork)
+       allocate(work(lwork), iwork(liwork) )
+       
+       call dsyevd( 'V', 'U', mo_tot_num, F,                         &
+           size(F,1), diagonal_Fock_matrix_mo,                       &
+           work, lwork, iwork, liwork, info)
+       
+       if (info /= 0) then
+         print *,  irp_here//' failed : ', info
+         stop 1
+       endif
+
+       call dgemm('N','N',ao_num,mo_tot_num,mo_tot_num, 1.d0, &
+         mo_coef, size(mo_coef,1), F, size(F,1), &
+         0.d0, eigenvectors_Fock_matrix_mo, size(eigenvectors_Fock_matrix_mo,1))
+       deallocate(work, iwork, F)
+
+  endif
 
-   deallocate(work, iwork, F, S)
 END_PROVIDER
  
 BEGIN_PROVIDER [double precision, diagonal_Fock_matrix_mo_sum, (mo_tot_num)]