Theoretical Analysis of Vibrational Lineshapes from Molecular Dynamics

The conventional spectral method for extracting anharmonic phonon properties from molecular dynamics (MD) requires prohibitively long simulations as the fitting function relies on the infinite time approximation. To that end, we derived the spectral lineshapes for arbitrary simulation lengths, while retaining the frequency shift and lifetime as fitting parameters. The theory was illustrated for graphene, hexagonal boron nitride, and silicon at the density functional theory (DFT) level, with up to nearly a factor of nine reduction in the required simulation time to reach convergence in the vibrational properties as compared to the standard approach. Such improvement in the convergence is expected in general provided the phonon anharmonicity is sufficiently weak, resulting in well-defined renormalized phonon quasiparticles. Application of the proposed approach has the potential to be far reaching as the theory applies equally well to ab initio MD based on DFT, time-dependent DFT dynamics, and parameterized force-fields and is thus expected to have important impact on topics ranging from strongly-correlated materials with sophisticated treatment of electron-electron interactions to biological systems.