quack/src/QuAcK/Bethe_Salpeter.f90

subroutine Bethe_Salpeter(TDA,singlet_manifold,triplet_manifold,eta, & 
                 nBas,nC,nO,nV,nR,nS,ERI,eW,eGW,OmRPA,XpY,XmY,rho,EcRPA,EcBSE)

! Compute the Bethe-Salpeter excitation energies

  implicit none
  include 'parameters.h'

! Input variables

  logical,intent(in)            :: TDA
  logical,intent(in)            :: singlet_manifold
  logical,intent(in)            :: triplet_manifold

  double precision,intent(in)   :: eta
  integer,intent(in)            :: nBas,nC,nO,nV,nR,nS
  double precision,intent(in)   :: eW(nBas)
  double precision,intent(in)   :: eGW(nBas)
  double precision,intent(in)   :: ERI(nBas,nBas,nBas,nBas)

  double precision              :: OmRPA(nS,nspin)
  double precision              :: XpY(nS,nS,nspin)
  double precision              :: XmY(nS,nS,nspin)
  double precision              :: rho(nBas,nBas,nS,nspin)

! Local variables

  integer                       :: ispin
  double precision,allocatable  :: OmBSE(:,:)

! Output variables

  double precision,intent(out)  :: EcRPA(nspin)
  double precision,intent(out)  :: EcBSE(nspin)

! Memory allocation

  allocate(OmBSE(nS,nspin))

!-------------------
! Singlet manifold
!-------------------

 if(singlet_manifold) then

    ispin = 1
    EcBSE(ispin) = 0d0

    call linear_response(ispin,.true.,TDA,.false.,eta,nBas,nC,nO,nV,nR,nS,1d0,eW,ERI, &
                         rho(:,:,:,ispin),EcRPA(ispin),OmRPA(:,ispin),XpY(:,:,ispin),XmY(:,:,ispin))
    call excitation_density(nBas,nC,nO,nR,nS,ERI,XpY(:,:,ispin),rho(:,:,:,ispin))

    OmBSE(:,ispin) = OmRPA(:,ispin)
    call linear_response(ispin,.true.,TDA,.true.,eta,nBas,nC,nO,nV,nR,nS,1d0,eGW,ERI, &
                         rho(:,:,:,ispin),EcBSE(ispin),OmBSE(:,ispin),XpY(:,:,ispin),XmY(:,:,ispin))
    call print_excitation('BSE         ',ispin,nS,OmBSE(:,ispin))

    ! Compute dynamic correction for BSE via perturbation theory

    call Bethe_Salpeter_dynamic_perturbation(TDA,eta,nBas,nC,nO,nV,nR,nS,OmRPA(:,ispin),OmBSE(:,ispin), & 
                                             XpY(:,:,ispin),XmY(:,:,ispin),rho(:,:,:,ispin))

  end if

!-------------------
! Triplet manifold
!-------------------

 if(triplet_manifold) then

    ispin = 2
    EcBSE(ispin) = 0d0

    call linear_response(ispin,.true.,TDA,.false.,eta,nBas,nC,nO,nV,nR,nS,1d0,eW,ERI, &
                         rho(:,:,:,ispin),EcRPA(ispin),OmRPA(:,ispin),XpY(:,:,ispin),XmY(:,:,ispin))
    call excitation_density(nBas,nC,nO,nR,nS,ERI,XpY(:,:,ispin),rho(:,:,:,ispin))

    OmBSE(:,ispin) = OmRPA(:,ispin)
    call linear_response(ispin,.true.,TDA,.true.,eta,nBas,nC,nO,nV,nR,nS,1d0,eGW,ERI, &
                         rho(:,:,:,ispin),EcBSE(ispin),OmBSE(:,ispin),XpY(:,:,ispin),XmY(:,:,ispin))
    call print_excitation('BSE         ',ispin,nS,OmBSE(:,ispin))

    ! Compute dynamic correction for BSE via perturbation theory

    call Bethe_Salpeter_dynamic_perturbation(TDA,eta,nBas,nC,nO,nV,nR,nS,OmRPA(:,ispin),OmBSE(:,ispin), & 
                                             XpY(:,:,ispin),XmY(:,:,ispin),rho(:,:,:,ispin))


  end if

end subroutine Bethe_Salpeter