Electronic Structure and Molecular Dynamics Simulations for Alkaline Earth and Alkali Metals




Chellathurai, Mazhalai

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The Naval Research Laboratory (NRL) Tight-Binding (TB) method, which was introduced by Cohen, Mehl, and Papaconstantopoulos [1], was applied successfully to all transition and noble metals by the same authors in 1996 [2]. In this work, the NRL-TB method has been applied for the alkaline earth metals Strontium (Sr) and Calcium (Ca), for which the authors of [2] had mixed success, and for the alkali metals Rubidium (Rb) and Lithium (Li), which were not included in [2] due to difficulties in handling very soft materials. The authors of [2] did not produce satisfactory results for the alkaline earth metals regarding elastic constants and phonon spectra, and did not present Molecular Dynamics (MD) simulations for these metals. Also, TB calculations for alkali metals were not attempted in [2]. In this disserta- tion, robust TB parametrizations have been obtained for Sr, Ca and Rb; also limitations of the method for Li have been presented. In trying to complement and improve on the TB results of [2], two problems were identified: (i) The first-principles Linear Augmented Plane Wave (LAPW) calculations based on the Generalized Gradient Approximation (GGA) pro- vide a more accurate input to the NRL-TB than the LAPW Local Density Approximation (LDA) used in [2], because it gives a better agreement with experiment for the equilibrium lattice parameter. (ii) In order to successfully perform MD simulations, the LAPW total energy inputs to the TB need to be extended to much smaller volumes than those considered in [2]. The central feature of this dissertation is about creating a good TB parametrization for the metals Sr, Ca, and Rb that accounts well for the band structure, and density of states, as well as producing accurate total energies for the evaluation of structural differ- ences, elastic constants, phonon frequencies and the MD derived quantities Mean Square Displacement (MSD) and Vacancy Formation Energy (VFE). Also, insights are presented for the application of the method to the very light element Li.



Alkaline earth metals, Alkali metals