Influence of electronic correlation on structural, dynamic, and elastic properties of Mg2Si

A wide variety of ab initio methods are used to study systematically the ef fects of electronic correlation on the charge density distribution, band st ructure, lattice parameters, phonon frequencies at the Brillouin zone cente r, and first-order elastic constants (C-ij) of Mg2Si. Two distinct atom-cen tered basis sets have been used: an all electron one, and another associate d with core pseudopotentials. Various levels of approximation have been tes ted: (i) HF (Hartree-Fock-Roothaan) calculations with no correlation energy correction; (ii) Hartree-Fock plus a posteriori correlation energy correct ion estimated using various DFT (density functional theory) correlation fun ctionals applied to the Hartree-Fock electronic density; (iii) DFT calculat ions with "exact" HF exchange and various DFT correlation functionals; (iv) "complete" DFT calculations in which both exchange and correlation are tre ated using a selection of exchange-correlation functionals. On the ground o f a comparison of the numerous data, complemented by plotted band structure and electronic density maps,the advantages and drawbacks of the representa tion of correlation within the four levels of approximation are discussed. Special focus is placed on the need for including correlation in the calcul ation of properties related to derivatives of the electronic potential ener gy surface of the crystal (phonon frequencies) and to electronically excite d states (band gap problem). Because of the peculiar partitions of exchange and correlation energies in DFT functionals, it is emphasized that the com bination of HF exchange with DFT correlation functionals is inadequate and may even worsen the HF errors on dynamical quantities involving inner atomi c displacements. In contrast, more satisfactory results are generally obtai ned carrying out complete exchange-correlation calculations.