Calculation of linear and nonlinear optical properties of azobenzene derivatives with Kohn-Sham and coupled-cluster methods

Linear polarizabilities (alpha) and second hyperpolarizabitities (gamma) of unsubstituted azobenzenes and 'push-pull' azobenzene derivatives are investigated using Kohn-Sham theory (KST) and coupled-cluster (CC) approaches. Various standard exchange-correlation functionals as well as a non-empirically 'tuned' long-range corrected (LC) functional with range-separated exchange are used in the KST calculations. When compared to correlated ab initio calculations and measurements, the tuned functional gives accurate tow-energy excitation energies, especially for charge transfer (CT) transitions, and performs welt for alpha. Basis set and solvent effects are also studied. In contrast to expectations, but in agreement with a prior study of pi-conjugated systems that do not have low-energy CT excitations, the improvements of the CT excitation energies for the push-pull pi-chromophores due to tuning do not go along with clear improvements of gamma toward the CC reference data, likely due to the importance of the dynamic electron correlation for this property.