First-principles local-density-functional (FP-LDF) and embedded-atom-method (EAM) total energies, for a single Pt atom adsorbed on Pt(111), are compared to each other and to field-ion-microscope (FIM) observations. Our EAM calculations predict a small barrier to diffusion and only a slight energy difference between adsorption in the fcc and hcp threefold hollows (both smaller than 1 meV). They correspond poorly to the results of the FP-LDF calculations, which imply a diffusion barrier of 0.38 eV for passage over a bridge, and a preference of 0.18 eV for the fcc against the hcp site. FIM measurements yield a diffusion barrier of (0.25\ifmmode\pm\else\textpm\fi{}0.02) eV, and imply that binding in the fcc site is preferred.