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compute s_z2_sz in diag_S_mat_elem

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Kevin Gasperich 2020-08-21 12:30:14 -05:00 committed by GitHub
parent 253ec57ae0
commit 770b4f6628
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1 changed files with 16 additions and 5 deletions
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21
src/determinants/s2.irp.f
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@ -8,24 +8,35 @@ double precision function diag_S_mat_elem(key_i,Nint)
BEGIN_DOC
! Returns <i|S^2|i>
END_DOC
integer :: nup, i
integer(bit_kind) :: xorvec(N_int_max)
integer :: nup, ntot, i
integer(bit_kind) :: xorvec(N_int_max), upvec(N_int_max)
do i=1,Nint
xorvec(i) = xor(key_i(i,1),key_i(i,2))
enddo
do i=1,Nint
xorvec(i) = iand(xorvec(i),key_i(i,1))
upvec(i) = iand(xorvec(i),key_i(i,1))
enddo
! nup is number of alpha unpaired
! ntot is total number of unpaired
nup = 0
ntot = 0
do i=1,Nint
if (xorvec(i) /= 0_bit_kind) then
nup += popcnt(xorvec(i))
ntot += popcnt(xorvec(i))
if (upvec(i) /= 0_bit_kind) then
nup += popcnt(upvec(i))
endif
endif
enddo
diag_S_mat_elem = dble(nup)
double precision :: sz
sz = nup - 0.5d0*ntot
!<S^2> = <S+ S-> + Sz(Sz-1)
diag_S_mat_elem = nup + sz*(sz-1)
end