Two-photon Absorption Strengths of Small- and Medium-Sized Molecules: Reference CC3 Values and Benchmarks

We present the largest dataset of highly-accurate vertical and degenerate two-photon transition strengths ($\delta^{\mbox{\tiny TPA}}$) for standard small- and medium-sized organic molecules, calculated using the quadratic response implementation of the third-order coupled cluster method that includes iterative triples (Q-CC3). The aug-cc-pVTZ basis set was used for all small molecules, while medium-sized molecules were assessed with aug-cc-pVDZ and the differences due to the basis sets are discussed. This dataset, encompassing 82 singlet transitions of various characters (Rydberg, valence, and double excitations), enables a comprehensive benchmark of both small basis sets and, alternative wavefunction methods when Q-CC3 calculations become beyond reach. These methods include quadratic (Q) response and equation of motion CCSD approximations, Q-CC2, second-order algebraic diagrammatic construction in its intermediate state representation (I-ADC2), as well as time-dependent density functional theory (TD-DFT) with a set of 5 commonly used exchange-correlation functionals. This extensive analysis provides a quantitative assessment of these methods, revealing how different system sizes, response intensities, and types of transitions affect their performances.