subroutine get_s2(key_i,key_j,phase,Nint)
 implicit none
 BEGIN_DOC
! Returns <S^2> 
 END_DOC
 integer, intent(in)  :: Nint
 integer, intent(in)  :: key_i(Nint,2)
 integer, intent(in)  :: key_j(Nint,2)
 double precision, intent(out) :: phase
 integer :: exc(0:2,2,2)
 integer :: degree
 double precision :: phase_spsm
 integer :: nup, i

 phase = 0.d0
 !$FORCEINLINE
 call get_excitation_degree(key_i,key_j,degree,Nint)
 select case (degree)
   case(2)
     call get_double_excitation(key_i,key_j,exc,phase_spsm,Nint)
     if (exc(0,1,1) == 1) then   ! Mono alpha + mono-beta
       if ( (exc(1,1,1) == exc(1,2,2)).and.(exc(1,1,2) == exc(1,2,1)) ) then
         phase =  -phase_spsm
       endif
     endif
   case(0)
      nup = 0
      do i=1,Nint
        nup += popcnt(iand(xor(key_i(i,1),key_i(i,2)),key_i(i,1)))
      enddo
      phase = dble(nup)
   end select
end