The iso-chemical shielding surface (ICSS) is a real-space function closely related to the wellknown NICS. It can unambiguously reveal the extent of magnetic shielding/deshielding effects originating from delocalized electrons in different regions of cyclic molecules. The isosurface maps of zz component of the ICSS (abbreviated as ICSS zz ) for C 16 displays a magnetic induction region protruding in the direction perpendicular to the ring plane, surrounded by a counteracting circular isosurface (Figure ). Specifically, the ICSS zz isosurface of C 16 in S 0 state is a picture in which the shielding region wraps the deshielding one (Figure ), while the T 1 and Q 1 states show opposite distribution patterns (Figures and). This confirms the conclusion obtained from AICD analysis that the ground state (S 0 state) and excited states (T 1 and Q 1 states) of C 16 are anti-aromatic and aromatic, respectively. Furthermore, compared to the T 1 state, Q 1 state presents a noticeably wider shielding/deshielding region, indicating a more significant aromaticity. For a clearer observation, contour lines from 20 to 120 ppm are bolded for each 20 ppm in color-filled contour maps of ICSS zz of the C 16 . It can be seen that the magnetic induction region of the Q 1 state is obviously larger than that of the T 1 state with the same value, which also shows more pronounced aromaticity in the Q 1 state from another perspective. For quantitative comparison purposes, we selected a cylindrical region located at the center of the carbon ring with a radius of 1 Å and endpoints ranging from (0, 0, -6) Å to (0, 0, 6) Å for evenly distributing integration points of ICSS zz (Figure ). The integral values of ICSS zz of the S 0 , T 1 , and Q 1 states are -5953.33, 4386.71, and 12923.1 ppm•Bohr 3 , respectively. Therefore, we can infer once again from that the ground state (S 0 state) of C 16 exists anti-aromaticity while the excited states (T 1 and Q 1 states) are aromatic, and the Q 1 state of C 16 have a much more pronounced aromaticity than T 1 state. These inferences are consistent with all aforementioned conclusions, and supplemented them with more rigorous quantitative measurements. The aromaticity strength of each electronic state of C 16 ring was further investigated by decomposing NICS(0) zz of various electronic states to obtain the contribution of π in orbitals, π out orbitals, and other orbitals to provide a deeper insight (Table ). The NICS(0) zz values of the overall π electrons of C 16 deduced from two different magnetic shielding calculation methods, that is, the continuous set of gauge transformations (CSGT) and the gauge-independent atomic orbital (GIAO), are quite consistent, which proves the reliability of the NICS(0) zz decomposition. It can be seen that the aromaticity characteristics of C 16 are basically derived from the contribution of π orbitals, while electrons on other orbitals have negligible contribution due to their high localization. For each electronic state, the contributions of π in and π out orbitals are similar; specifically, they both have a significant anti-aromaticity contribution to S 0 state, a certain aromaticity contribution to T 1 state, and a strong aromaticity contribution to Q 1 state. Of particular note is that the NICS(0) zz values for ground-state C 18 and benzene at the same calculation level are found to be -25.7 and -16.1 ppm, respectively, indicating that the Q 1 state of C 16 with NICS(0) zz of -106.1/-106.8 ppm is far more aromatic than both of them. Since aromaticity essentially comes from the delocalization of electrons on molecular rings, we further investigate the aromaticity differences of the three electronic states of C 16 from the perspective of electron density of delocalized bond (EDDB). The isosurface maps of the globally delocalized electrons of different electronic states of C 16 given by EDDB method and the corresponding electron numbers show that Q 1 and S 0 states have the richest and scarcest delocalized electron distributions, respectively, while the extent of delocalization of electrons in T 1 state occupies the medium-sized level (Figure ). The fully delocalized electron numbers in the T 1 and Q 1 states are about twice and three times that of the S 0 state, respectively, which further confirmed that the order of aromaticity of C 16 is Q 1 state > T 1 state > S 0 state. Clearly, the aromaticity analysis by EDDB based on electronic structure is consistent with the previous analysis based on magnetic properties. It can also be seen from the EDDB isosurface maps that the delocalized electrons on the C 16 ring come entirely from its π in and π out electrons, which are distributed evenly throughout the ring in similar extent, and this is the fundamental reason why π in and π out electrons contribute Exploring the nature of S 0 →T 1 and S 0 →Q 1 excitations is valuable for understanding the aromaticity differences of S 0 , T 1 and Q 1 states. In order to characterize them in terms of an intuitive orbital transition picture, biorthogonalization transformations are performed on the unrestricted Kohn-Sham molecular orbitals. In the biorthogonalized orbital (BO) representation of the T 1 and Q 1 states of C 16 , the spin density is almost entirely contributed by two and four singly occupied orbitals, respectively (Figure ), which are directly related to the electron excitations. Since S 0 and Q 1 states have 16 π in and 16 π out electrons and T 1 state has 15 π in and 17