Relaxation of hcp(0001) surfaces: A chemical view

First-principles calculations predict 7.8% and 6.3% contractions of the outermost layer spacings of Ti(0001) and Zr(0001). Charge smoothing, slight at close-packed metal surfaces, cannot explain such large relaxations. Bond-order bond-length correlation is a more promising concept. Bonds to undercoordinated Ti or Zr should be unusually short, given the small ratio, similar to 0.7, of dimer bond length to nearest-neighbor distance for these elements.