calc_pgen_ueg Function

    function calc_pgen_ueg(ilutI, ex, ic) result(pgen)
        ! i also have to write a pgen recalculator for the pgens with this
        ! new UEG excitation generator.. i am a bit confused why this has
        ! not been done yet i have to admit..
        ! and i need this function if i want to use it with HPHF..
        integer, intent(in) :: ex(2, 2), ic
        integer(n_int), intent(in) :: ilutI(0:niftot)
        real(dp) :: pgen

        real(dp) :: p_elec, p_orb, cum_arr(nBasis), cum_sum
        integer :: src(2), orb_a, tgt(2)

        if (ic /= 2) then
            pgen = 0.0_dp
            return
        end if

        src = get_src(ex)
        tgt = get_tgt(ex)

        ! i should return 0 if b < a.. since those excitations
        ! are never created.. but are the actually called ever? hm..
        ! the question is how does the ex() determination work actually?
        ! is it ever possible in the HPHF case eg. that b > a when
        ! comparing two dets? hm.. check!
!         if (tgt(1) > tgt(2)) then
!             pgen = 0.0_dp
!             return
!         end if

        ! now the real part..
        p_elec = 1.0_dp / real(ElecPairs, dp)

        orb_a = tgt(1)

        ! i need to recalct the cum_arr
        call create_ab_list_ueg(ilutI, src, cum_arr, cum_sum)

        if (cum_sum < EPS) then
            pgen = 0.0_dp
            return
        end if
        ! and now i have to check orba probability
        if (orb_a == 1) then
            p_orb = cum_arr(1) / cum_sum
        else
            p_orb = (cum_arr(orb_a) - cum_arr(orb_a - 1)) / cum_sum
        end if

        pgen = p_orb * p_elec

    end function calc_pgen_ueg