Improved SCF computations on the benzene molecule and especially the benzenium ion (protonated benzene), using a large contracted gaussian basis set including polarization functions, are presented. The results for the benzenium ion are consistent with the conventional model of a C5H5+ structure with an attached CH2 group, the whole somewhat modified and stabilized by hyperconjugation. The calculations predict the equilibrium HCH angle at the protonated carbon atom to be 103.5° and that the distance to the adjacent carbon atom is lengthened by 0.055 Å. The proton affinity is found to be 189 kcal/mol as compared with an experimental value of 183.1. One-electron properties are compared with corresponding benzene values, and changes at the ring positions are discussed. A population analysis for both σ and π (and quasi-π) overlap populations and charges is reported and discussed. From the overlap populations in the ring bonds, it is estimated that the lateral C-C bonds are shortened by 0.015 Å, and the C-C bonds para to the CH2 group are lengthened by 0.020 Å, as compared with benzene. © 1976, American Chemical Society. All rights reserved.