QuAcK/src/eDFT/UCC_lda_exchange_energy.f90

subroutine UCC_lda_exchange_energy(nEns,wEns,nCC,aCC,nGrid,weight,rho,Cx_choice,doNcentered,Ex)

! Compute the unrestricted version of the curvature-corrected exchange functional

  implicit none
  include 'parameters.h'

! Input variables

  integer,intent(in)            :: nEns
  double precision,intent(in)   :: wEns(nEns)
  integer,intent(in)            :: nCC
  double precision,intent(in)   :: aCC(nCC,nEns-1)
  integer,intent(in)            :: nGrid
  double precision,intent(in)   :: weight(nGrid)
  double precision,intent(in)   :: rho(nGrid)
  integer,intent(in)            :: Cx_choice
  logical,intent(in)            :: doNcentered

! Local variables

  integer                       :: iG
  double precision              :: r

  double precision              :: a1,b1,c1,w1
  double precision              :: a2,b2,c2,w2
  double precision              :: Fx1,Fx2,Cx

! Output variables

  double precision              :: Ex

! Single excitation parameter

!  a1 = 0.0d0
!  b1 = 0.0d0
!  c1 = 0.0d0

! Parameters for H2 at equilibrium

! a2 = +0.5751782560799208d0
! b2 = -0.021108186591137282d0
! c2 = -0.36718902716347124d0

! Parameters for stretch H2

!  a2 = + 0.01922622507087411d0
!  b2 = - 0.01799647558018601d0
!  c2 = - 0.022945430666782573d0

! Parameters for He

! a2 = 1.9125735895875828d0
! b2 = 2.715266992840757d0
! c2 = 2.1634223380633086d0

! Parameters for He N -> N-1

  a1 = aCC(1,1)
  b1 = aCC(2,1)
  c1 = aCC(3,1)

! Parameters for He N -> N+1

  a2 = aCC(1,2)
  b2 = aCC(2,2)
  c2 = aCC(3,2)

! Fx1 for states N and N-1
! Fx2 for states N and N+1

  if(doNcentered) then 
    
     w1 = wEns(2)
     Fx1 = 1d0 - w1*(2d0 - w1)*(a1 + b1*(w1 - 1d0) + c1*(w1 - 1d0)**2)
   
     w2 = wEns(3)
     Fx2 = 1d0 - w2*(2d0 - w2)*(a2 + b2*(w2 - 1d0) + c2*(w2 - 1d0)**2)

  else  


    w1 = wEns(2)
    Fx1 = 1d0 - w1*(1d0 - w1)*(a1 + b1*(w1 - 0.5d0) + c1*(w1 - 0.5d0)**2)

    w2 = wEns(3)
    Fx2 = 1d0 - w2*(1d0 - w2)*(a2 + b2*(w2 - 0.5d0) + c2*(w2 - 0.5d0)**2)

  endif

  select case (Cx_choice)

    case(1)
      Cx = CxLSDA*Fx1

    case(2)
      Cx = CxLSDA*Fx2

    case(3)
      Cx = CxLSDA*Fx2*Fx1

    case default
      Cx = CxLSDA

  end select

! Compute GIC-LDA exchange energy

  Ex = 0d0

  do iG=1,nGrid

    r = max(0d0,rho(iG))

    if(r > threshold) then
      Ex = Ex + weight(iG)*Cx*r**(4d0/3d0) 
    endif

  enddo

end subroutine UCC_lda_exchange_energy