Dresden 2017 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 52: Electronic-Structure Theory: New Concepts and Developments in Density Functional Theory and Beyond - V (joint session DS, HL, MA, MM, O, TT, organized by O)

TT 52.2: Talk

Wednesday, March 22, 2017, 15:15–15:30, GER 38

Anharmonic Vibrations in Solids: Why and When Going Beyond Perturbative Treatments is Necessary — •Hagen-Henrik Kowalski, Maja-Olivia Lenz, Christian Carbogno, and Matthias Scheffler — Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin

In ab initio theory, the nuclear motion is typically assessed using a truncated second order Taylor expansion for the potential energy (harmonic approximation). Recent computational and methodological advancements [1] allow to extend this expansion to the third order so to approximately treat also anharmonic effects. Little is known, however, about the role of higher order terms. In this contribution, we systematically compare how a third-order expansion performs with respect to techniques that are able to capture higher degrees of anharmonicity, e.g., the quasi-harmonic approximation and fully anharmonic molecular dynamics. For this purpose, anharmonic properties such as the thermal expansion and the Grüneisen parameters are computed for a set of materials with increasing degree of anharmonicity (Si, Mg₂Si, CuCl, and ZrO₂). This reveals that a third order expansion can still lead to quantitative and even qualitative errors at elevated temperatures and/or in highly anharmonic systems. Eventually, we discuss the impact of the chosen exchange-correlation functionals on these calculations and the implications of these findings for the computation of thermal conductivities [2].

[1] D. A. Broido, et al., Appl. Phys. Lett. 91, 231922 (2007).

[2] C. Carbogno, R. Ramprasad, and M. Scheffler, ArXiv: 1608.06917.