This paper reports an investigation of the phenomenon of specific adsorption of halide ions on a Cu(100) surface using Monte Carlo simulations. The system was modeled by considering each ion in a water lamina placed between two copper walls. The potentials used in simulations were constructed by fitting to results of quantum calculations. The solvent contribution to the potential of mean force (pmf) was calculated for each of the four halide ions using the free energy perturbation method. Given the difficulty of finding a reliable ion-metal potential, several alternatives, representing extremal models, were tested in combination with the solvent mean force on the ions, F-, Cl-, Br- or I-. The results for the pmf on an ion near the metal surface are discussed in the light of the experimental data available. The sensitivity of the results to the type of ion-metal potential used in the simulations is stressed.