**Thermal expansion in insulating solids from first principles**

ET Ritz and SJ Li and NA Benedek, JOURNAL OF APPLIED PHYSICS, 126, 171102 (2019).

DOI: 10.1063/1.5125779

In this Tutorial, we describe the use of the quasiharmonic approximation and first-principles density functional theory (DFT) to calculate and analyze the thermal expansion of insulating solids. We discuss the theory underlying the quasiharmonic approximation and demonstrate its practical use within two common frameworks for calculating thermal expansion: the Helmholtz free energy framework and the Gruneisen theory. Using the example of silicon, we provide a guide for predicting how the lattice parameter changes as a function of temperature using DFT, including the calculation of phonon modes and phonon density of states, elastic constants, and specific heat. We also describe the calculation and interpretation of Gruneisen parameters, as well as how they relate to coefficients of thermal expansion. The limitations of the quasiharmonic approximation are briefly touched on, as well as the comparison of theoretical results with experimental data. Finally, we use the example of ferroelectric PbTiO3 to illustrate how the methods used can be adapted to study anisotropic systems. Published under license by AIP Publishing.

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