Periodic DFT calculations are used to study the effect of a homogeneous electric field applied perpendicular to a Pt(111) surface on the bond distances, binding energies, and vibrational frequencies of atop- and fcc-adsorbed CO at various coverages. The observed structural and energetic modifications can be understood in terms of modest field-induced charge transfer between charged metal surface and adsorbate and are well-described by classical first and second-order Stark models. Electronic differences between atop and fcc adsorption cause CO in these sites to respond differently to applied fields. After correcting for the GGA site preference error, CO adsorption is predicted to shift from atop to fcc at potentials <-0.19 V A(-1). The results are in qualitative agreement with previously reported cluster-based DFT models but differ quantitatively due to difference in modeled coverage, surface relaxation, and finite size effects. The calculated 44.4 cm(-1) V(-1) A shift in C-O stretch frequency with electric field (Stark tuning rate) compares favorably with UHV experiments but is significantly lower than the value obtained in electrochemical measurements, highlighting the importance of adsorbate environment on the magnitude of the tuning rate. The calculated coverage dependence of the tuning rate is in good agreement with previous UHV experiments.