The presence of SHG in RP perovskites made with PEA and TPMA spacer cations but not BA cations indicates that the bulky PEA and TPMA cations template different bonding in the inorganic lead iodide network. Single-crystal X-ray diffraction structures show that the TPMA n = 2, TPMA n = 3, and PEA n = 3 2D-RP perovskites have noncentrosymmetric space groups (see Table ). The previously reported SCXRD structure refinements of BA n = 2 and n = 3 2D-RP perovskites showed that they could either be centrosymmetric or noncentrosymmetric space groups, but a weak SHG response, I SHG /I THG ≈ 0.01, was measured with a single excitation color which was used as the basis to conclude that BA n = 2 and n = 3 are noncentrosymmetric. Our multidimensional harmonic generation experiments did not reveal SHG in any of the BA n = 2 and n = 3 crystals we examined. We further quantify the limit of our ability to measure SHG (Figure ) and find that our detection limit is three orders of magnitude lower than the limit needed to observe SHG at the previously reported SHG to THG ratio. It is possible that local symmetry-breaking effects were at play which led to some weak SHG signal, as noted by the authors. It is also possible that mPL photons from low energy "edge" states were measured and attributed to SHG. The lack of SHG in our multidimensional measurements on BA n = 2 and n = 3 crystals indicates that these crystal structures are better described as centrosymmetric in the Ccmm and Acam space groups, respectively, instead of the previously assigned space groups of Cc2m and C2cb. The main difference between the centrosymmetric and non-centrosymmetric structures is whether the methylammonium cations are disordered or ordered, respectively. Based on previous SCXRD measurements, the PEA n = 2 structure is believed to be centrosymmetric. However, this crystal structure (originally reported in 1991) was based on a partial structural model with incomplete organic cation atom positions, likely due to their high degree of disorder at the 298 K experiment temperature. We therefore grew high-quality single crystals (see Figure ) of the PEA n = 2 phase and carried out SCXRD analysis at 100 K. Our complete structural refinement yielded a centrosymmetric space group with a well-defined PEA cation disorder. A structural refinement was also attempted in a non-centrosymmetric group, however the same PEA cation disorder was observed and numerical indicators suggested a higher symmetry (centrosymmetric) space group. Since our structural refinement confirms the centrosymmetry of PEA n = 2, it is tempting to attribute the observed SHG from PEA n = 2 to surface effects or localized symmetry breaking domains. To investigate this contradiction between observing SHG and a centrosymmetric structure from SCXRD, we examined many PEA n = 2 crystals from different synthesis batches and observed SHG in all of them (data shown in Figure and Figure ). Some samples displayed a greater SHG to THG ratio than others, indicating inhomogeneity among the crystals. In an attempt to mitigate local defects, we also grew PEA n = 2 using an alternative, slow, organic solvent based growth method. The resultant crystals also supported SHG. We intentionally degraded PEA n = 2 crystals in a humidity chamber and observed significantly more mPL from the n = 2 phase and the emergence of mPL from the n = 1 phase that was formed upon degradation. Crucially, we did not observe a significant increase in SHG efficiency when a sample had its surface degraded. This observation suggests that surface effects are not the dominant contributor to such observed SHG.