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It has been discussed the optimization of the valence force field model for non-ideal III-nitrides wurtzite materials. To describe the static and dynamical lattice properties of nitrides within the same model, the anisotropy of the wurtzite structure and additional second-nearest-neighbor atomic interaction terms are included in the lattice strain energy expression. The sets of the force constants have been optimized through an iterative scheme in which calculated lattice properties are fitted to experimentally reported data. The model has been applied to the binary wurtzite compounds (GaN, InN) and random InxGa1-xN alloys. The calculated phonon dispersions, elastic constants and internal strain parameters of GaN and InN are in reasonable agreement with the experimental/ab initio values.