Noncentral forces and equilibrium of a hexagonal lattice

An electron gas model incorporating noncentral forces is used to study the lattice dynamics of thallium and zinc. The dynamical matrix is split into two parts: (i) The short-range (s.r.) part and (ii) The long-range (l.r.) part. The short-range part is evaluated by considering Taylor's expansion of the crystal potential energy in the harmonic approximation and the long-range part is calculated on the basis of the screened Coulomb potential. The computed phonon dispersion relations and the Debye characteristic temperatures for Tl and Zn have been compared with the available experimental data