1. Summary
This repository contains the key raw data discussed in the manuscript:

J. M. Skelton, L. A. Burton, S. C. Parker, A. Walsh, C.-E. Kim, A. Soon, J. Buckeridge, A. A. Sokol, C. R. A. Catlow, A. Togo and I. Tanaka , “Anharmonicity in the High-Temperature Cmcm Phase of SnSe: Soft Modes and Three-Phonon Interactions”, Physical Review Letters 117, 075502 (2016), DOI: 10.1103/PhysRevLett.117.075502

The article has been published with “gold” open access and is available free of charge from the journal website, accessible from the DOI resolver landing page.

2. Overview
The files in this repository provide the main raw and processed data necessary to reproduce the calculations and analysis presented in the paper. Any data not available in this repository may be requested from the authors.
The data includes a set of input files for the Vienna Ab initio Simulation Package (VASP) electronic-structure code,[1] together with input and output files for the Phonopy and Phono3py packages[2, 3] used to set up and post-process the lattice-dynamics calculations. Users are referred to the comprehensive documentation on these codes available online for further information:

•	VASP: https://www.vasp.at/ (accessed 30/09/2016)
•	Phonopy: http://atztogo.github.io/phonopy/ (accessed 30/09/2016)
•	Phono3py: http://atztogo.github.io/phono3py/ (accessed 30/09/2016)

We note that we have not included the projector augmented-wave (PAW) pseudopotentials[4, 5] with the VASP input files, as these were taken from the pseudopotential databases distributed under licence as part of the code. The Sn and Se pseudopotentials used for the calculations were: Sn - “PAW_PBE Sn_d 06Sep2000”, Se - “PAW_PBE Se 06Sep2000”.

3. Available Data
The data is collected into three GZip-compressed TAR archives, one containing sample input files and two containing data from the calculations on the Pnma and Cmcm phases of SnSe. The archives can be unpacked using the tar utility on macOS and Linux, or the free PeaZip software on Windows (http://www.peazip.org/, accessed 30/09/2016).
The contents of the archives are outlined in the following subsections. We note that within some archives a small number of files have been duplicated across groups of input and output files in order to keep the groups self-contained.

3.1 Sample input files
This archive contains a minimal set of input files for the VASP calculations and Phonopy post processing performed for the Pnma and Cmcm phases of SnSe.
VASP INCAR and KPOINTS files for the structural relaxations, the single-point calculations to evaluate the second and third-order force-constant matrices, and the single-point calculations used to map the potential-energy surfaces along the imaginary modes are provided. The POTCAR file may be reconstructed from the pseudopotential databases distributed with the code (“PAW_PBE Sn_d 06Sep2000”, Se - “PAW_PBE Se 06Sep2000”).
The Phonopy configuration files provide example settings for computing the phonon density of states (DoS) curves and band dispersions of the two phases of SnSe from the calculated second-order force constants.

3.2 Raw data from the calculations on Pnma SnSe
This archive contains the optimised structure (in the VASP POSCAR format; “SnSe-Pnma.vasp”) along with input and output from the Phonopy and Phono3py calculations described in the paper, stored under the “Phonopy” and “Phono3py” subdirectories, respectively.
For further information about the content and format of these files, readers are referred to the documentation of the respective codes (see Section 2). For the Phono3py calculation, the calculated lattice thermal conductivity tensors as a function of temperature have been extracted from the HDF5 files and written to a CSV-format file for convenience (“kappa-m161616.csv”).
In addition to the main Phonopy calculation, a series of calculations were performed with different supercells to test convergence of the supercell size (see the ESI associated with the paper for details); the data from these calculations is contained in the “Phonopy_SC-Convergence” subdirectory.

3.3 Raw data from the calculations on Cmcm SnSe
	This archive contains the same data as for the Pnma phase (see Section 3.2, above), together with data from phonon calculations performed at a series of unit-cell volumes and from the imaginary-mode renormalisation procedure described in the paper.
Data from the volume-dependent phonon calculations, including the shape- relaxed structures, is contained in the “Phonopy_Volume” subdirectory. Each POSCAR file and subdirectory is suffixed with the volume of the conventional unit cell in Å3, with the volume of the optimised equilibrium geometry being 204 Å3.
In this archive, the “Phonopy_SC-Convergence” subdirectory contains data from the calculations performed with optimised supercells to obtain more accurate phonon band dispersions along certain paths in the phonon Brillouin zone (as described in the paper and ESI), in addition to data from the same supercell-size convergence tests as were performed for the Pnma phase (see Section 3.2).
Data from the calculations utilising the imaginary-mode renormalisation procedure developed in the paper is stored in the “Phonopy_Renormalisation” and “Phono3py_Renormalisation” subdirectories. The former includes: (1) the mapped 1D and 2D potential-energy surfaces along the two imaginary modes, (2) fits of the 1D potentials to polynomial functions, (3) the calculated energy levels and temperature-dependent effective renormalised harmonic frequencies associated with the double-wells, (4) thermodynamic functions corrected with the renormalised frequencies, (4) and complete harmonic-phonon calculations performed with the second-order force constants corrected using the 0, 300, 800 and 1000 K renormalised frequencies. The latter contains input and output files from Phono3py calculations performed with the second-order force constants corrected using the 300 and 800 K renormalised frequencies.

4. References
[1] G. Kresse and J. Hafner, “Ab initio molecular dynamics for liquid metals”, Physical Review B 47, 558(R) (1993)
[2] A. Togo and I. Tanaka, “First principles phonon calculations in materials science”, Scripta Materialia 108, 1 (2015)
[3] A. Togo, L. Chaput and I. Tanaka, “Distributions of phonon lifetimes in Brillouin zones”, Physical Review B 91, 094306 (2015)
[4] P. E. Blochl, “Projector augmented-wave method”, Physical Review B 50, 17953 (1994)
[5] G. Kresse and D. Joubert, “From ultrasoft pseudopotentials to the projector augmented-wave method”, Physical Review B 59, 1758 (1999)