The π-electron dipole polarizabilities and hyperpolarizabilities of organic conjugated hydrocarbons with oddmembered carbon cycles (nonalternant systems) are calculated using local semiempirical coupled-cluster theory. The peculiarity of the approach is the use of local ethylene π-molecular orbitals to represent double bonds in the molecule (cue-CCSD approach). The size-limit values of the (hyper)polarizabilities for different types of nonalternant oligomers obtained in the calculations show a significant dependence on the molecular structure. The effect of the electron correlation in the hyperpolarizability calculations is investigated using different levels of the coupled-cluster method including the simplest CCSD model and other models up to the model where the correlation effects due to all four electron excitations are accounted for, as well as with the full configuration interaction model.
Calculations of electronic excitation energies, electron charge distribution, and optical parameters for π-conjugated oligomeric systems are carried out with a new local semiempirical π-electron variant of the coupled clusters (CC) theory involving single and double excitations. The results obtained in the calculations demonstrate adequate accuracy of the proposed linear-response generalization of the local semiempirical π-electron CC approach employing covalently nonbonded molecules of ethylene as the basis set in the calculation. Limit values corresponding to the infinite system size for the lowest excitation energies for polyenes and for condensed and noncondensed polyfulvenes are obtained.
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