Third-order perturbation theory and exchange and correlation in the lattice dynamics of indium

The lattice dynamics of indium is studied based on pseudopotential theory. It is found that experimental results for neutron scattering, elastic constants, and specific heat are well fitted by using the nonlocal optimized model potential of Shaw to second order in the pseudopotential and by setting the effective mass equal to 0.8. Calculations with local pseudopotentials to second order do not fit as well. Corrections for exchange and correlation increase the frequencies of a given mode but do not change the overall shape of the dispersion curves. The inclusion of third-order in the pseudopotential according to the method of Bertoni, Bortolani, Calandra, and Nizzoli and according to modifications of their method lead to significant changes in the dispersion curves with all modes along symmetry directions being unstable. (author)