Theoretical study of the surface structure and vibrations of solid argon

The influence of zero-point oscillations on the equilibrium structures and the vibrational modes for the (001), (110) and (111) surfaces of solid Ar is investigated at zero temperature. The surface structures are determined by minimizing the total energy, the static lattice energy plus zero-point energy, where the zero-point energy is evaluated from Einstein frequencies. It is found that the interlayer distances near the surface are larger than the values for bulk argon and are also larger than the distances predicted by minimizing only the static lattice energy. Based upon the surface structures determined by each of these two different minimization schemes, the phonon dispersion curves have been calculated for 21 layer slabs. The qualitative features of the dispersion curves in each case are very similar but there is a significant overall reduction in the frequencies calculated using the structures determined by minimizing the total energy, including the zero-point contribution. The rms vibrational amplitudes of atoms near the surface are calculated from the Einstein frequencies and their values relative to the bulk amplitude are the same as those evaluated by Allen and De Wette Phys. Rev. 179 (1969) 873] from the lattice dynamics of slabs.