AB-INITIO CALCULATIONS OF THE COHESIVE, ELASTIC, AND DYNAMICAL PROPERTIES OF COSI2 BY PSEUDOPOTENTIAL AND ALL-ELECTRON TECHNIQUES

Ab initio calculations of cohesive properties, elastic constants, and phonon dispersions for CoSi2 were performed by means of the Vienna ab initio molecular-dynamics package (VAMP), which makes use of ultrasoft pseudopotentials. In addition, the all-electron full-potential linear ized augmented-plane-wave method was used for the calculation of equil ibrium properties as well as elastic constants derived from second der ivatives of total energies. For both methods, total energies, their de rivatives, and related quantities are fully converged with respect to basis sizes and number of k points. Our results for C-44 prove that at omic positions in the strained crystal have to be fully relaxed in ord er to get quantitatively useful results. The results of both methods o btained within the same type of local-density approximation are in ver y good agreement between each other. Because elastic constants are ver y sensitive quantities for an ab initio method in general, the reprodu ction of high-quality all-electron data by VAMP demonstrates the power of suitably constructed pseudopotentials even for systems containing transition elements. VAMP was also applied to calculate elastic consta nts in two other ways, namely, directly from the stress-strain relatio ns (which yielded the same results as obtained from total-energy deriv atives) and from acoustic branches of the phonon dispersion. In this c ase, however, we only succeeded in getting a useful result for C-44 wh ereas for the remaining two elastic constants prohibitively large supe rcells would be needed. Finally, VAMP calculations were performed with in the framework of the generalized gradient approximation to density- functional theory. From that, data were derived which are in very good agreement with experimental values.