  Density-functional calculations on the polar TiX(111) (X = C, N) surfaces
show (i) for clean surfaces, strong Ti3d-derived surface resonances (SR's) at
the Fermi level and X2p-derived SR's deep in the upper valence band and (ii)
for adatoms in periods 1-3, pyramidic trends in atomic adsorption energies,
peaking at oxygen (9 eV). A concerted-coupling model, where adatom states
couple to both kinds of SR's in a concerted way, describes the adsorption. The
chemisorption versatility and the general nature of the model indicate
ramifications and predictive abilities in, e.g., growth and catalysis.
