vertex_not_allowed Function

    pure function vertex_not_allowed(n_elec_1, n_elec_3, orb, elec, ras) result(not_allowed)

        integer, intent(in) :: n_elec_1, n_elec_3
        integer, intent(in) :: orb, elec
        type(ras_parameters), intent(in) :: ras
        integer :: n_elec_2
        logical :: not_allowed

        not_allowed = .true.

        n_elec_2 = tot_nelec - n_elec_1 - n_elec_3

        ! For the current (orb, elec) vertex to be allowed, it must lie within one of three
        ! trapeziums, defined by the RAS parameters. The first corresponds to RAS1 orbitals,
        ! the second to RAS2 orbitals and the third to RAS3 orbitals. The three if statements
        ! below correspond to these three cases, and the if statements within to the
        ! trapeziums which they must lie in.

        if (orb <= ras%size_1) then
            ! If a RAS1 orbital.
            ! Condition for (orb,elec) combination to be allowed.
            if (orb - elec <= ras%size_1 - n_elec_1 .and. orb - elec >= 0 &
                .and. elec <= n_elec_1) not_allowed = .false.

        else if (orb > ras%size_1 + ras%size_2) then
            ! If a RAS3 orbital.
            if (orb + (tot_nelec - elec) <= tot_norbs .and. &
                orb + (tot_nelec - elec) >= tot_norbs - (ras%size_3 - n_elec_3) .and. &
                elec >= (n_elec_1 + n_elec_2)) not_allowed = .false.

        else
            ! If a RAS2 orbital.
            if (orb - (elec - n_elec_1) <= ras%size_1 + (ras%size_2 - n_elec_2) .and. &
                orb - (elec - n_elec_1) >= ras%size_1 .and. &
                elec >= n_elec_1 .and. elec <= (n_elec_1 + n_elec_2)) not_allowed = .false.

        end if

    end function vertex_not_allowed