The vibrational frequencies of a 15-layer slab are calculated for a simple shell model of lithium fluoride, and the surface modes occurring are analyzed. The static relaxations of a slab and of a single surface are calculated for the same model, and the effects of the relaxation on the vibrational frequencies are investigated. A puckering distortion of the surface layer is incorporated in the vibrational calculations for the first time and is found in this case to introduce one extra surface mode.

• Received 7 March 1983

DOI:https://doi.org/10.1103/PhysRevB.28.3124