  We characterized CO2 adsorption and diffusion on the missing row
reconstructed Cu(100)-O surface using a combination of scanning tunneling
microscopy (STM) and density functional theory (DFT) calculations with
dispersion. We deposited CO2 molecules in situ at 5K, which allowed us to
unambiguously identify individual CO2 molecules and their adsorption sites.
Based on a comparison of experimental and DFT-generated STM images, we find
that the CO2 molecules sit in between the O atoms in the missing row
reconstructed Cu(100)-O surface. The CO2 molecules are easily perturbed by the
STM tip under typical imaging conditions, suggesting that the molecules are
weakly bound to the surface. The calculated adsorption energy, vibrational
modes, and diffusion barriers of the CO2 molecules also indicate weak
adsorption, in qualitative agreement with the experiments. A comparison of
tunneling spectroscopy and DFT-calculated density of states shows that the
primary change near the Fermi level is associated with changes to the surface
states with negligible contribution from the CO2 molecular states.
