Monocyclic conjugated molecules have stabilities, bond lengths, and magnetic properties following expectations from Hückel's Rule. Two hydrocarbons which seem to deviate from these generalizations, s-indacene and cyclohepta [def]fluorene, were studied with density functional theory. The former has characteristics expected for both aromatic and antiaromatic molecules; the latter is a ground state triplet. Potentially aromatic transition states with 10 electrons were also investigated. The mechanism of the 5,5-sigmatropic shift is predicted to be stepwise involving diradical intermediates.On the occasion of this splendid ISNA-9, it is my distinct pleasure and opportunity to summarize recent work on the electronic structures of novel aromatic systems, including our assessment of the nature of the transition states of pericyclic reactions involving 10 electrons. This presentation is divided into three parts: (i) a summary of our work [1,2] on the computational methods to obtain geometries and properties of aromatic and nonaromatic systems; (ii) a description of our new results [3,4] on density functional theory studies of several novel -systems which deviate from normal properties in several ways; and (iii) a summary of our studies of transition states of 5,5-sigmatropic shifts where we found that the aromatic concerted transition states which are common in six-electron pericyclic reactions are less favored than stepwise pathways involving diradical intermediates [5].Of the many criteria for aromaticity proposed during this century, three have proven to be the most characteristic measures of aromaticity: energy, geometry, and magnetic properties. There is no necessary relationship between the three. Kinetic stability is a function of the energies of transition states for reactions, and thermodynamic stability depends upon the reference molecules. Bond lengths usually alternate between single and double for nonaromatic or antiaromatic systems, while aromatic systems have nearly equal bond lengths. Magnetic properties arise from the diamagnetic ring currents of aromatic systems.Schleyer and coworkers showed that there are very good linear correlations between geometric, energetic, and magnetic properties of five-membered ring heterocycles [6]. For an extensive set of these heterocycles, the various criteria are in good agreement, providing a particularly straightforward interpretation of the electronic structures and properties of these molecules.We have found that density functional theory, with the B3LYP functional and a relatively modest basis set (6-31G*) provides very good geometries and properties of the bridged-[14]annulenes [1,2]. For other aromatic systems, similar conclusions have been made by other researchers [6,7]. We have now applied density functional theory to study two very interesting unsaturated hydrocarbons, which we will show to deviate substantially from expectations based on monocyclic systems.The tricyclic hydrocarbon, s-indacene, 1, is a 12 electron system. Although this electron cou...