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https://github.com/LCPQ/quantum_package
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35 lines
872 B
Fortran
35 lines
872 B
Fortran
subroutine get_s2(key_i,key_j,phase,Nint)
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implicit none
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BEGIN_DOC
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! Returns <S^2>
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END_DOC
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integer, intent(in) :: Nint
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integer, intent(in) :: key_i(Nint,2)
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integer, intent(in) :: key_j(Nint,2)
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double precision, intent(out) :: phase
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integer :: exc(0:2,2,2)
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integer :: degree
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double precision :: phase_spsm
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integer :: nup, i
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phase = 0.d0
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!$FORCEINLINE
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call get_excitation_degree(key_i,key_j,degree,Nint)
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select case (degree)
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case(2)
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call get_double_excitation(key_i,key_j,exc,phase_spsm,Nint)
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if (exc(0,1,1) == 1) then ! Mono alpha + mono-beta
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if ( (exc(1,1,1) == exc(1,2,2)).and.(exc(1,1,2) == exc(1,2,1)) ) then
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phase = -phase_spsm
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endif
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endif
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case(0)
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nup = 0
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do i=1,Nint
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nup += popcnt(iand(xor(key_i(i,1),key_i(i,2)),key_i(i,1)))
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enddo
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phase = dble(nup)
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end select
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end
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