To aid in spectral assignments, we used DFT+U to model frequently discussed Cu dimer structures (Figure ) and calculate their Raman spectra (Figure ). For each optimized structure, the change in polarizability was calculated along the normal modes of the simulated vibrations to determine the frequencies of Raman-active vibrations (see Section 2.4 for full details). The precise assignment of a complex whose calculated molecular vibrations describe the Raman features in Figures ande., at 603 cm -1 ) requires that several criteria be met. First, the CuxOy complex must exist as Cu(II), which excludes the possibility of bis-µoxo dicopper and trimeric Cu-oxo complexes. Second, the CuxOy intermediate must absorb 532 nm (18,800 cm -1 ; Figures and) light to provide the resonance Raman effect. Third, the candidate dimer structures modeled must possess Raman scattering features within 20 cm -1 of the experimentally measured vibrations, which is the range of deviations between experimental and theoretical frequencies commonly reported for similar materials. Fourth, the simulated vibration near 603 cm -1 must not contain significant Cu-O or O-O bond deformations (e.g., stretching modes of superoxo or peroxo complexes), as these vibrations would produce a significant Δ 18 O2.