get_pgen_pick_hole_from_active_space Function

    function get_pgen_pick_hole_from_active_space(this, ilut, srcGASInd, spin_idx) result(pgenVal)
        class(GAS_disc_ExcGenerator_t), intent(in) :: this
        integer(n_int), intent(in) :: ilut(0:NIfD)
        integer, intent(in) :: srcGASInd, spin_idx

        real(dp) :: pgenVal
        integer :: nEmpty
        nEmpty = this%GAS_size(srcGASInd) &
                 - sum(popCnt(iand(ilut(0:NIfD), &
                                   this%GAS_spin_orbs(0:NIfD, srcGASInd, spin_idx) &
                                  ) &
                             ) &
                      )
        ! if the excitation is not possible, pgen is void
        if (nEmpty == 0) then
            pgenVal = 1.0_dp
        else
            ! adjust pgen
            pgenVal = 1.0_dp / real(nEmpty, dp)
        end if
    end function get_pgen_pick_hole_from_active_space