Linear polarizabilities and second hyperpolarizabilities of streptocyanines: Results from broken-Symmetry DFT and new CCSD(T) benchmarks.

We present a detailed analysis of the linear polarizability (α) and second hyperpolarizability (γ) in a series of streptocyanines, as predicted with various range-separated functionals and CCSD(T)-based methods. Contrary to previous work on these systems, we find that the lowest-energy electronic states for the larger streptocyanine oligomers are not closed-shell singlets, and improved accuracy can be obtained with certain DFT methods by allowing the system to relax to a lower-energy broken-symmetry solution. Our extensive analyses are complemented by new large-scale CCSD(T) and explicitly correlated CCSD(T)-F12 calculations that comprise the most complete and accurate benchmarks of α and γ for the streptocyanine systems to date. Taken together, our CCSD(T) and broken-symmetry DFT calculations (1) show that the MP2 benchmarks used in previous studies still exhibit significant errors (~ 25% for α and ~100% for γ) and, therefore, the MP2 calculations should not be used as reliable benchmarks for polarizabilities or hyperpolarizabilities, and (2) emphasize the importance of testing for a lower-energy open-shell configuration when calculating nonlinear optical properties for these systems. © 2018 Wiley Periodicals, Inc.