This study evaluated the performance of several computational methods and basis sets for calculating the vertical two-photon transition strengths (𝛿 TPA ) of singlet excited-state in small-and medium-sized molecules, using Q-CC3 as a benchmark. The analysis underscored the importance of employing a not too compact basis set. Specifically, benchmarks on small molecules using the aug-cc-pVTZ basis set indicated that the aug-cc-pVDZ basis set introduces an error of approximately 10%. Using aug-cc-pVDZ as a reference, we observed that 6-31+G(d) reduce the performances up to 24%, thus advising against the use of such small basis sets for performing reference calculations.