Cohesive Energies of Be and Mg Chalcogenides
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Mat. Res. Soc. Symp. Proc. Vol. 491 0 1998 Materials Research Society
tools, even comparisons of theoretical predictions obtained by different methods can be fully meaningful. In this preliminary report we shall confine ourselves to zincblende and rocksalt structures, and to the calculation of cohesive energies, bulk moduli and equilibrium volumes. Moreover all calculations have been performed with a minimal basis set plus an extra set of p orbitals. Further properties and crystal structures, together with the extension to BeTe and MgTe phases, will be taken into account in a forthcoming paper[6], where an optimization of the basis will be carried out. THE DISCRETE VARIATIONAL METHOD DVM has been originally developed in the seventies by Ellis and Painter[7] to perform band structure calculations, its basic idea being to capitalize on the high efficiency of a numerical representation of localized orbitals to make their use competitive against plane waves expansions. In the case of solid state calculations this attempt has been effectively frustrated by the advent of norm conserving pseudopotentials[8], and DVM has since evolved into a fullfledged first-principles method specialized to deal with clusters[4,9]. We have undertaken the job of taking one of the latest versions[ 10] and converting it to an object-oriented architecture by embedding most of its Fortran instructions into a C++ framework[1 1]. In this way we have been able to retain the optimisation of the key numerical routines of the original code while enjoying the flexibility of the object-oriented C++ language. The advantages of this new architecture has greatly eased the subsequent transitions, again to solid state calculations and to W. Yang's order N approach[ 12]. Our computational procedure is thus based on a DVM code which straightforwardly implements all-electron density-functional calculations within LDA. We have used the Von Barth-Hedin formulation of the exchange and correlation contribution. The wavefunctions are expanded into a set of orbitals, obtained by solving numerically the isolated atom problem, and a grid of 2000 points/atom is used in numerical integrations in the unit cell. The use of such a high density grid has been made necessary by the convenience[6] of using the highly efficient diophantine algorithm even in the framework of integrations over unit cells (instead of the whole space). We have used the frozen core approximation to avoid spurious energy minimization effects on our structural properties. The cohesive energy has been computed for sets of molecular volumes for both structures of each compond and the results have been fitted to the Murnaghan equation of state. RESULTS AND DISCUSSION Our cohesive energies per atom versus the atomic volume, for zincblende and rocksalt structures, are shown in Figs. l(a)-l(c) and in Figs. l(d)-l(f) for Be and Mg chalcogenides, respectively. The curves predict that of the two structures zincblende is the most stable one for Be compounds, while the reverse should happen for Mg compounds.
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