For a small size ring, the out-plane system retains its shoulder-to-shoulder bonding feature, nevertheless, the in-plane system may behave a mixed feature of σand π-bonding. Taking C6 as the smallest case, Figure shows the natural orbitals (NO) involved in the in-plane and out-plane systems, each of which has four energy levels. The difference between two systems is obvious. The four energy levels in in-plane system hold degeneracy of 1, 2, 1 and 2, respectively; while for out-plane system, these are 1, 2, 2 and 1. NOs of in-plane system not only interact with each other through shoulder-to-shoulder overlap, but also exhibit notably head-to-head overlap inside the C6 ring. Therefore, it would be more appropriate to describe in-plane system of C6 as a mix σand π-covalent system, instead of a π-conjugated system. It is obvious that NO-14 exhibits a high level of symmetry with π-conjugated system surrounding the whole ring and shares multicenter σ-bonding feature. This multicenter feature should be described as a six-centered interaction, compared with its pairing orbital NO-19. For the degenerate orbitals of NOs-17 and 18, π-conjugated system has one node plane across the molecule. Both of them have two groups of 1,3-σ-bonding overlaps and one 1,4-σ*-anti-bonding overlap, sharing the same node plane with π-system. In parallel, NOs-22 and 23 are π*-conjugated system with two node planes, possessing one group of 1,4-σ*-bonding overlap in each NO. Such phenomenon is also revealed by Mayer bond order analysis. As can be seen from Table , 1,3-interaction in C6 evaluated by Mayer bond order is 0.143 and 1,4-interaction is 0.088. However, comparing with same results obtained in benzene (Table ), we may find that for ideal sim-membered π-conjugated system, π-bonding has no contribution to 1,3-interaction and therefore the Mayer bond order of 0.143 in C6 can only be attributed to 1,3-σ-bonding. At the same time, with two π-conjugated systems, 1,4-interaction intensity in C6 (0.069) is even slightly weaker than that in benzene (0.071), which is in parallel to 1,4-σ-anti-bonding overlaps observed in NOs. Mixing σ-feature into the π-conjugated system could greatly disturb the orbital order. Therefore, degeneracies of 1, 2, 1 and 2 on four energy levels are observed in the out-plane system of C6, which is significantly different from the conventional diagram of 6-centered π-conjugated system, for which the out-plane system of C6 is actually a good example.