Cálculos de primeiros princípios para BaO.

Abstract

In this work we present first principles calculations with Density Functional Theory, DFT, using norm-conserving pseudopotentials. Initially, we perform calculations of structural properties for metallic barium, Ba, in the Local Density Approximation, LDA, and Generalized Gradient Approximation, GGA, for the exchange and correlation potential. This study aimed at familiarization with the formalism and with the program SIESTA, which was used throughout this dissertation. The GGA turned out to perform out better than LDA, yielding results close to experimental values. After that, we perform structural calculations for Barium Oxide, BaO, in its four allotropic forms, in the LDA and GGA. A study of the energetics of structural phase transitions in this system was carried out in order to identify the sequence of structural transitions. We find that LDA is unable to determine the correct sequence of structural transitions, while GGA reproduces it correctly. Finally, we calculated the phase transition pressures using the enthalpy method. These phase transition pressures, at T = 0 K, are in reasonable agreement with ultrasoft pseudopotential calculations. Our study shows that the use of GGA is fundamental for an adequate description of the energetics of phase transitions in BaO.