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https://github.com/pfloos/quack
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75 lines
1.3 KiB
Fortran
75 lines
1.3 KiB
Fortran
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subroutine Ec_AC(ispin,nBas,nC,nO,nV,nR,nS,ERI,XpY,EcAC)
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! Compute the correlation energy via the adiabatic connection formula
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implicit none
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include 'parameters.h'
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! Input variables
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integer,intent(in) :: ispin
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integer,intent(in) :: nBas,nC,nO,nV,nR,nS
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double precision,intent(in) :: ERI(nBas,nBas,nBas,nBas)
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double precision,intent(in) :: XpY(nS,nS)
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! Local variables
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integer :: i,j,a,b
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integer :: ia,jb,kc
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double precision :: delta_spin
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double precision,allocatable :: P(:,:)
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! Output variables
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double precision,intent(out) :: EcAC
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! Singlet or triplet manifold?
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delta_spin = 0d0
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if(ispin == 1) delta_spin = +1d0
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if(ispin == 2) delta_spin = -1d0
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! Memory allocation
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allocate(P(nS,nS))
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! Compute P = (X+Y)(X+Y) - 1
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P(:,:) = 0d0
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do ia=1,nS
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do jb=1,nS
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do kc=1,nS
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P(ia,jb) = P(ia,jb) + XpY(ia,kc)*XpY(kc,jb)
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enddo
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enddo
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P(ia,ia) = P(ia,ia) - 1d0
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enddo
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! Compute Tr[VP]
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EcAC = 0d0
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ia = 0
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do i=nC+1,nO
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do a=nO+1,nBas-nR
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ia = ia + 1
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jb = 0
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do j=nC+1,nO
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do b=nO+1,nBas-nR
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jb = jb + 1
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EcAC = EcAC + (1d0 + delta_spin)*ERI(i,b,a,j)*P(jb,ia)
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enddo
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enddo
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enddo
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enddo
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end subroutine Ec_AC
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