Dielectric matrix tail

It is clear from Table II that the convergence of the phonon frequencies with respect to the dimension of e(q,6,G ) is relatively slow. To include effects of higher reciprocal lattice vectors one can add a diagonal "tail", i.e. a diagonal part, to the dielectric matrix [12] ... 

The results for the phonon frequencies of Silicon obtained by the present authors are shown, for wave vectors in r-. A- and l-directions, in figure 5. In this calculation both the tail in the dielectric matrix and the extrapolation in the Hamiltonian were used. A meaningful overall agreement with experiment is obtained. The discrepancy, averaged over all calculated branches, is 14%. Generally the acoustic branches are in better agreement with experiment than the optical modes. 

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