quack/src/RPA/linear_response_C_pp.f90

subroutine linear_response_C_pp(ispin,nBas,nC,nO,nV,nR,nOO,nVV,e,ERI,C_pp)

! Compute the C matrix of the pp channel

  implicit none
  include 'parameters.h'

! Input variables

  integer,intent(in)            :: ispin
  integer,intent(in)            :: nBas,nC,nO,nV,nR,nOO,nVV
  double precision,intent(in)   :: e(nBas),ERI(nBas,nBas,nBas,nBas) 
  
! Local variables

  double precision              :: eF
  double precision,external     :: Kronecker_delta

  integer                       :: a,b,c,d,ab,cd

! Output variables

  double precision,intent(out)  :: C_pp(nVV,nVV)

! Define the chemical potential
 
  eF = e(nO) + e(nO+1)
! eF = 0d0

! Build C matrix for the singlet manifold

  if(ispin == 1) then

    ab = 0
    do a=nO+1,nBas-nR
     do b=a,nBas-nR
        ab = ab + 1
        cd = 0
        do c=nO+1,nBas-nR
         do d=c,nBas-nR
            cd = cd + 1
 
            C_pp(ab,cd) = + (e(a) + e(b) - eF)*Kronecker_delta(a,c)*Kronecker_delta(b,d) &
                          + (ERI(a,b,c,d) + ERI(a,b,d,c))/sqrt((1d0 + Kronecker_delta(a,b))*(1d0 + Kronecker_delta(c,d)))
 
          end do
        end do
      end do
    end do

  end if

! Build C matrix for the triplet manifold, or alpha-alpha block, or in the spin-orbital basis

  if(ispin == 2 .or. ispin == 4) then

    ab = 0
    do a=nO+1,nBas-nR
     do b=a+1,nBas-nR
        ab = ab + 1
        cd = 0
        do c=nO+1,nBas-nR
         do d=c+1,nBas-nR
            cd = cd + 1
 
            C_pp(ab,cd) = + (e(a) + e(b) - eF)*Kronecker_delta(a,c)*Kronecker_delta(b,d) & 
                          + ERI(a,b,c,d) - ERI(a,b,d,c)
 
          end do
        end do
      end do
    end do

  end if

! Build the alpha-beta block of the C matrix

  if(ispin == 3) then

    ab = 0
    do a=nO+1,nBas-nR
     do b=nO+1,nBas-nR
        ab = ab + 1
        cd = 0
        do c=nO+1,nBas-nR
         do d=nO+1,nBas-nR
            cd = cd + 1
 
            C_pp(ab,cd) = + (e(a) + e(b) - eF)*Kronecker_delta(a,c)*Kronecker_delta(b,d) & 
                          + ERI(a,b,c,d)
 
          end do
        end do
      end do
    end do

  end if

end subroutine linear_response_C_pp