A finite-field method for the calculation of polarizabilities and hyperpolarizabilities is developed based on both a n energy expansion and a dipole moment expansion. This procedure is implemented in the MOPAC semiempirical program. Values and components of the dipole moment ( p ) , polarizability ( a ) , first hyperpolarizability (p), and second hyperpolarizability ( 7 ) are calculated as a n extension of the usual MOPAC run. Applications to benzene and substituted benzenes are shown as test cases utilizing both MNDO and AM1 Hamiltonians.

High levels of ab initio (MP2, CASSCF, CASPT2N) and density functional (Becke3LYP) theory have been used to assess the homoaromatic character of some strained semibullvalenes. Based on geometric, energetic, and magnetic criteria (magnetic susceptibility exaltations and the nucleus independent chemical shifts, NICS), C s semibullvalene itself is not aromatic, but the C 2 v transition state for its Cope rearrangement is highly bishomoaromatic. Appropriate annelations destabilize the C s geometries, and the bishomoaromatic structures are the only minima for several semibullvalenes:  1,5-methano; 2,8:4,6-bisethano and -bismethano; and 2,8-monoethano. In contrast, the 4,6-ethanosemibullvalene is predicted to be localized and not homoaromatic. Inclusion of dynamic electron correlation is very important for computing the geometries and relative energies of the delocalized structures. To aid experimental investigations, the UV and 13C NMR spectra of some of the semibullvalenes have been predicted. Long-wavelength UV absorptions and down-field 13C NMR chemical shifts for C2,8,4,6 are characteristic.

No abstract