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Quantum Mechanical Hartree-Fock Study of the Elastic Properties of Li,S and Na,S BY A. LICHANOT (a), E. APRA (b), and R. DOVES (b)The binding energy and elastic constants of Li,S and Na,S are investigated at an ab initio all electron Hartree-Fock level. The comparison between calculated and experimental values leads to satisfactory agreement for Li,S and a substantial difference in the Na,S case. The comparative evolution of the elastic constants in the series of alkaline oxides and sulphides with the size of the cation seems to indicate that the discrepancy is mainly due to an overestimation of the experimental elastic constants of Na,S.

Structural properties of the (NaCl-type) and (CsCl-type) phases of alkaline-earth oxides and their phase transition have been investigated with the periodic ab initio linear combination of atomic orbitals method implemented in the CRYSTAL program. The geometries have been optimized and the bulk modulus evaluated. The calculations have been done at the Hartree-Fock (HF) and density functional theory (DFT) levels. In this last case, the exchange-correlation potential correcting the electronic density uses either one local or three non-local models. The comparison of the different approaches allows us to identify a trend, in order to obtain results in better agreement with experiment.