  Using a lattice-gas model with pairwise interactions, we study the ordered
structures, coverage dependence of the heat of adsorption, and other
experimentally observable behavior of adsorbed CO overlayers on Pd(100) single
crystal surfaces. Transfer matrix and Monte Carlo methods give accurate
information regarding the lattice-gas model that often contradicts simple
mean-field-like analysis. We demonstrate the usefulness of the model by
reproducing experimental results over a large range of pressures and
temperatures.
