CO is an important probe molecule in infrared spectroscopy 1 , which describes local electrostatic and chemical environments in heterogeneous catalysis and in electrochemical characterization 2 of transition metal (TM) active sites (CO-stripping voltammetry). Moreover, CO is a key reactant-product in CO2 reduction to low-carbon fuels and high value products such as alcohols, acids, and hydrocarbons and in carbon sequestration for reducing greenhouse gases. However, our theoretical understanding of CO adsorbed on a TM surface is fragmented. This is largely because our main tool to model such systemsdensity functional theory (DFT) even with functionals tuned for catalysis-lacks sufficient fidelity to reliably describe either the total energy or spectral properties, as we show in some detail in this work. As one example, many functionals strongly over-bind CO at the hollow site on TM surfaces. Such uncertainty hampers the informative and predictive nature of theoretical studies on CO2 reduction. How CO initially binds to the substrate is an essential component in the initial step for processes such as CO2 splitting into CO* and O*. If already the site preference of the initial step of the reaction process cannot be resolved to the required accuracy, modeling the entire reaction process remains in doubt. That being said, even while we find DFT energies are not reliable, we will show that DFT predictions of the structure (bond lengths) are much less variable between functionals, and align much better with available experimental data.