Coupled-cluster approach for studying the electronic and nonlinear optical properties of conjugated molecules

We develop a coupled-cluster theory with single and double excitations in the framework of the equation-of-motion approach, to study the electronic excitations and linear and nonlinear optical properties of conjugated oligomers: the linear polyenes and paraphenylenevinylenes. It is found that this approach can provide extremely high accuracy in calculating the ground-state and ionic excited-state properties. The accuracy is relatively poor for the even-parity covalent excited states ${(2A}_g$ for instance) since the molecular-orbital basis is not appropriate to describe such spin excitations. The linear and nonlinear optical coefficients are calculated via the correction vector method. We stress that special attention should be paid to the secular terms, which have been either neglected or improperly included in previous studies for the third-order molecular polarizability; within a single and double excitation framework, it is imperative to fully include this term; however, at the single configuration interaction or random-phase approximation level, this term should be deliberately excluded.