gen_excits Subroutine

    subroutine gen_excits(nI, n_excits, det_list, ex_flag)
        integer, intent(in) :: nI(nel)
        integer, intent(out) :: n_excits
        integer(n_int), intent(out), allocatable :: det_list(:, :)
        integer, optional, intent(in) :: ex_flag
        character(*), parameter :: this_routine = "gen_all_excits_default"

        integer :: n_singles, n_doubles, n_dets, ex(2, maxExcit), ex_flag_
        integer :: nJ(nel)
        logical :: tpar, found_all
        integer(n_int) :: ilut(0:niftot)
        integer, parameter :: arbitrary_number = 42 ! can be neither 1 or 2

        n_excits = -1

        call EncodeBitDet(nI, ilut)

        if (present(ex_flag)) then
            ASSERT(present(ex_flag) .implies. any(ex_flag == [1, 2]))
        end if

        ! If it is set to neither 1 nor 2, all excitations
        ! are generated
        def_default(ex_flag_, ex_flag, arbitrary_number)

        ! for reference in the "normal" case it looks like that:
        call CountExcitations3(nI, ex_flag_, n_singles, n_doubles)

        n_excits = n_singles + n_doubles

        allocate(det_list(0:niftot, n_excits))
        n_dets = 0
        ex = 0
        call GenExcitations3(nI, ilut, nJ, ex_flag_, ex, tpar, found_all, &
                             .false.)

        do while (.not. found_all)
            n_dets = n_dets + 1
            call EncodeBitDet(nJ, det_list(:, n_dets))

            call GenExcitations3(nI, ilut, nJ, ex_flag_, ex, tpar, &
                                 found_all, .false.)
        end do

        if (n_dets /= n_excits) then
            write(stdout, *) "expected number of excitations: ", n_excits
            write(stdout, *) "actual calculated ones: ", n_dets
            call stop_all(this_routine, "Incorrect number of excitations found")
        end if

        ! Sort the dets, so they are easy to find by binary searching
        call sort(det_list, ilut_lt, ilut_gt)

    end subroutine gen_excits