Optical parameters of Π-conjugated oligomer chains from the semiempirical local coupled-cluster theory

The Π-electron semiempirical local coupled-cluster theory has been developed and used to calculate molecular optical parameters (polarizabilities and hyperpolarizabilities) of fragments of conjugated polymers. The method is based on using molecular Π-orbitals of ethylene as an orbital basis for the conjugated systems. The method is termed the Covalent Unbonded molecules of Ethylene, cue. Based on the comparison of the calculations performed with higher levels of theory (especially with the full configuration interaction method), it has been demonstrated that for selected conjugated molecules the approach is accurate and capable of reproducing the available experimental data with good accuracy. The cue-CC results show qualitatively correct dependency on the sizes of Π-systems.