sltcnd_2_base_ua Function

    function sltcnd_2_base_ua(nI, ex, tSign, assert_occupation) result(hel)
        ! Calculate the  by the Slater-Condon Rules when the two
        ! determinants differ by two orbitals exactly (the simplest case).
        integer, intent(in) :: nI(nel)
        type(Excite_2_t), intent(in) :: ex
        logical, intent(in) :: tSign
        logical, intent(in), optional :: assert_occupation
        HElement_t(dp) :: hel
        debug_function_name("sltcnd_2_base_ua")

#ifdef DEBUG_
    block
        use constants, only: stderr
        use util_mod, only: stop_all
        logical :: test_occupation

        if (present(assert_occupation)) then
            test_occupation = assert_occupation
        else
            test_occupation = .true.
        end if

        if (test_occupation) then
            if (.not. occupation_allowed(nI, ex)) then
                write(stderr, *) 'src', ex%val(1, :)
                write(stderr, *) 'tgt', ex%val(2, :)
                write(stderr, *) 'nI', nI
                call stop_all(this_routine, "Not allowed by occupation.")
            end if
        end if
    end block
#endif

        ! Only non-zero contributions if Ms preserved in each term (consider
        ! physical notation).
        hel = sltcnd_2_kernel_ua(ex)

        if (tSign) hel = -hel
    end function sltcnd_2_base_ua