# Dresden 2020 – wissenschaftliches Programm

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# HL: Fachverband Halbleiterphysik

## HL 11: Frontiers in Electronic-Structure Theory - Focus on Electron-Phonon Interactions I (joint session O/CPP/DS/HL)

### HL 11.8: Vortrag

### Montag, 16. März 2020, 12:30–12:45, GER 38

Assessment of Approximate Methods for Anharmonic Free Energies — •Venkat Kapil^{1}, Edgar Engel^{2}, Mariana Rossi^{3}, and Michele Ceriotti^{1} — ^{1}Swiss Federal Institute of Technology, Switzerland — ^{2}Department of Physics, University of Cambridge, UK — ^{3}MPI for Structure and Dynamics of Matter, Hamburg, Germany

Quantitative estimations of thermodynamic stabilities, measured by free energies, must take into account thermal and quantum zero-point nuclear motion. While these effects are easily estimated within a harmonic approximation, corrections arising from the anharmonic nature of the interatomic potential are often crucial and their accurate computations require expensive path integral simulations. Consequently, different approximate methods for computing affordable estimates of anharmonic free energies have been developed. Understanding which of the approximations involved are justified for a given system is complicated by the lack of comparative benchmarks. We here assess the accuracy of some of the commonly used approximate methods: vibrational self-consistent field and self-consistent phonons by comparing anharmonic corrections to Helmholtz free energies against reference path integral calculations. We study a diverse set of systems, ranging from simple weakly anharmonic solids to flexible molecular crystals with freely-rotating units and conclude that efforts towards obtaining computationally-feasible anharmonic free-energies of molecular systems must focus at reducing the expense of path integral methods.
*Kapil, Venkat, et al. Assessment of Approximate Methods for Anharmonic Free Energies. JCTC, 2019, doi:10.1021/acs.jctc.9b00596.*