Probing anharmonic phonons by quantum correlators A path integral approach

Probing anharmonic phonons by quantum correlators: A path integral approach

  • 06 Mar 2021
  • .

Materials Science (2021)

Authors: Tommaso Morresi, Lorenzo Paulatto, Rodolphe Vuilleumier, Michele Casula

We devise an efficient scheme to determine vibrational properties from Path Integral Molecular Dynamics (PIMD) simulations. The method is based on zero-time Kubo-transformed correlation functions and captures the anharmonicity of the potential due to both temperature and quantum effects. Using analytical derivations and numerical calculations on toy-model potentials, we show that two different estimators built upon PIMD correlation functions fully characterize the phonon spectra and the anharmonicity strength. The first estimator is associated with force-force quantum correlators and gives access to the fundamental frequencies and thermodynamic properties of the quantum system. The second one is instead connected to displacement-displacement correlators and probes the lowest-energy phonon excitations with high accuracy. We also prove that the use of generalized eigenvalue equations, in place of the standard normal mode equations, leads to a significant speed-up in the PIMD phonon calculations, both in terms of faster convergence rate and smaller time-step bias. Within this framework, using ab initio PIMD simulations, we compute phonon dispersions of diamond and of the high-pressure I41/amd phase of atomic hydrogen. We find that, in the latter case, the anharmonicity is stronger than previously estimated and yields a sizeable red-shift in the vibrational spectrum of atomic hydrogen.

Subjects:    Materials Science (cond-mat.mtrl-sci); Computational Physics (physics.comp-ph)
Cite as:      arXiv:2103.04094 [cond-mat.mtrl-sci]
                  (or arXiv:2103.04094v2 [cond-mat.mtrl-sci] for this version)