Validity of the quasiharmonic analysis for surface thermal expansion of Ag(111)

For temperatures above 0.6T(m) (the bulk melting temperature) we show that the quasiharmonic approximation leads to increasingly larger values of the surface thermal expansion of Ag(1 1 1) as compared to that obtained from molecular dynamics simulations based on fully anharmonic interaction potentials. The inadequacy of the quasiharmonic approximation is traced to the excessive softening of the surface phonon frequencies. We discuss the validity of the quasiharmonic approximation for surfaces at elevated temperature, in view of recent x-ray scattering data on Ag(1 1 1).