Density functional theory calculations are commonly used in combination with other methods like a supercells and small displacements or perturbation theory for the description of lattice dynamics of various materials. These methods are working well if the crystal lattice of the material is nearly harmonic at low temperatures and they can be extended to obtain the free energy at elevated temperatures through quasiharmonic approximation. However, the situation becomes dramatic when the anharmonic term is not small or a dynamical instability as seen by appearance of imaginary vibrational frequencies is present at zero temperature. PdH system is such a system where conventional methods fail. In this work, we calculate the full phonon spectra of palladium hydride from ab initio molecular dynamic by means of temperature dependent effective potential technique (TDEP) proposed and developed by Hellman et al. [1, 2]. Our results are in good agreement with available experimental data. However, we observe certain mismatch with theoretical results obtained by a recently proposed alternative technique, SSCHA [3, 4].
[1] O. Hellman, I. A. Abrikosov, and S. I. Simak, Physical Review B 84, 180301 (2011).
[2] O. Hellman, P. Steneteg, I. A. Abrikosov, and S. I. Simak, Physical Review B 87, 104111 (2013).
[3] I. Errea, M. Calandra, and F. Mauri Physical Review L 111,177002 (2013)
[4] I. Errea, M. Calandra, and F. Mauri, Physical Review B 89, 064302 (2014)