The stretching frequency of CO adsorbed on Pt(1 1 1), vc-0, changes if an external electric field F is applied (the vibrational Stark effect). The slope dv(c-o)/dF measured in electrochemical cells, is a factor of two larger than that obtained in UHV experiments. The reasons for this discrepancy are still being debated, whereas existing DFT calculations "support" the electrochemical value. This study, on the contrary, produces a slope close to that found in UHV. The difference between our ab initio calculations and previous studies is that the latter were performed with finite clusters, whereas here the electric field is applied to a periodic supercell. This highlights the importance of the computational setup, and in particular the boundary conditions, for calculations of charged systems. (c) 2007 Elsevier B.V. All rights reserved.