The concept of multiple bonding between metals dates back to the late 1920s with the first structural confirmation in 1964. Over the last 60 years, multiple bonds between a wide variety of transition metals have been assigned bond orders ranging from single to sextuple. Nowadays, synthetic chemists seek out new metal-metal bonds for as catalysts, building blocks in metalorganic frameworks, photosensitizers, and molecular conductors. Group 6 metal-metal bonds have been broadly studied for their interesting bonding and unique photophysical properties. Of these, Cr-Cr multiple bonds are notable because of their rich electronic structure and unique spectroscopic properties. Starting with Cotton's work in the 1970s, a variety of dichromium complexes have been synthesized; characterization by diffraction has shown that the bond distances range from 1.7 to 2.3 Å. Although in the early days formal bond orders were estimated based on distances, computational work using multiconfigurational methods has established that the electronic structure of these complexes is multiconfigurational due to the small energy splitting between the σ, π, and δ orbitals. The concept of an effective bond order (EBO), that accounts for partial occupation of low-lying antibonding orbitals, has been used to quantify the nature of the overall Cr-Cr bond that is composed of multiple partial bonds. Furthermore, these computations highlight that the multiconfigurational electronic structure of Cr 2 bonds is more nuanced compared to Mo 2 and W 2 analogues. As a result, the Cr-Cr bond order often does not correlate with bond distance. Note that for some particularly short Cr-Cr distances, density functional theory (DFT) has been used successfully since the electronic structure can be represented by fully occupied bonding orbitals. The lack of correlation between bond distance and bond order motivated a number of vibrational studies of these compounds; diffraction studies alone cannot be used to predict bonding or, in turn, vibrational force constants. Da Re et al. emphasize the unique challenges in dichromium complexes by comparing the empirical force constants and bond distances for first and second row transition metals. For example, Mo(II)-Mo(II) bond distances empirically correlate to their vibrational frequencies, as expected, with force constants increasing monotonically with decreas-ing bond distance. Cr(II)-Cr(II) bonds deviate from the norm and have multiple modes in the low-frequency region of the spectra involving the Cr centers, making peak assignment challenging. The measured force constants in two complexes with ultrashort Cr-Cr bonds, Cr 2 (mhp) 4 and Cr 2 (dmp) 4 , drew our interest (mhp = deprotonated 6-methyl-2-hydroxypyridine and dmp = 2,6dimethoxyphenyl). In the former, the Cr-Cr stretching mode, ν(Cr 2 ), was first assigned at 556 cm -1 by Cotton et al. but was later revised to be at 340 (or 400) cm -1 by The Cr-Cr bond in Cr 2 (dmp) 4 is even shorter, but the ambiguity surrounding the vibrational mode remains. An estimation based on the aggregate isotope shift would lead to the assignment at 650 cm -1 ; however, an alternative assignment, based on the average of three low-frequency bands assigned to include motion in the Cr-Cr internal coordinate, would put ν(Cr 2 ) closer to 365 cm -1 . In both complexes, the larger value is consistent with the force constant that would be predicted from a simple harmonic approximation based on the bond distance. The lower values would suggest a significantly weaker bond than one would expect from a formally quadruple bond. Vibrational analysis of compounds consisting of a few dozen atoms is routinely performed by DFT. However, as mentioned earlier, the electronic structure of dichromium complexes is highly multiconfigurational, which DFT cannot describe properly. For the computation of bond length, several procedures have been proposed to empirically correct DFT simulations. For example, DFT geometry optimizations can be performed keeping the Cr-Cr bond distance fixed at the experi-mental value with subsequent analysis of the electronic structure with CASPT2. Alternatively, a series of constrained DFT optimizations can be performed to determine the Cr-Cr bond distance point-wise with CASPT2 yielding good agreement with experiment. These hybrid approaches, however, are not applicable to the computation of the vibrational spectra. Moreover, full CASPT2 geometry optimization for complexes with transition metals had only been performed on small molecules, and even then not routinely. Here, we directly apply the CASPT2 electronic structure method to the prediction of vibrational spectra of Cr-Cr complexes, in which the vibrational frequencies and normal modes are computed from the second derivative of CASPT2 energies. Full geometry optimizations with the CASPT2 method and subsequent harmonic vibrational analysis are made possible by utilizing the recently developed CASPT2 analytical gradient program and extending it to compute second-order energy derivatives by means of finite difference of the first derivatives. Anharmonic effects are known to be important from experimental work on these complexes; therefore, the anharmonicity of the Cr-Cr harmonic stretching mode is studied.