  We study Bromine and Chlorine chemisorption on a Ag(100) surface, using a
lattice-gas model and the quantum-mechanical Density Functional Theory (DFT)
method. In this model the Br and Cl ions adsorb at the fourfold hollow sites of
the Ag(100) surface, which can be represented by a square lattice of adsorption
sites. Five different coverages were used for each kind of adsorbate. For each
adsorbate and coverage, we obtained the minimum-energy configuration, its
energy, and its charge distribution. From these data we calculated dipole
moments, lateral interaction energies, and binding energies. Our results show
that for Br the lateral interactions obtained by fitting to the adsorption
energies obtained from the DFT calculation are consistent with long-range
dipole-dipole lateral interactions obtained using the dipole moments calculated
from the DFT charge distribution. For Cl we found that, while the long-range
dipole-dipole lateral interactions are important, short-range attractive
interactions are also present. Our results are overall consistent with
parameter estimates previously obtained by fitting room-temperature Monte Carlo
simulations to electrochemical adsorption isotherms [I.\ Abou Hamad et al., J.\
Electroanal. Chem.\ 554 (2003), 211; Electrochim.\ Acta 50 (2005), 5518].
