High-Throughput Phonon Calculations from Ab Initio Molecular Dynamics Using the Real-Space Multigrid Method (RMG)

The real-space multigrid-based electronic structure code RMG (www.rmg.org) is highly parallel with excellent scalability to thousands of nodes and GPUs. It has accurate forces and is very suitable for high-throughput, large-scale ab initio molecular dynamics (AIMD). The AIMD results can be used to extract force constants that include anharmonic and finite temperature effects, and thus to compute phonon band structures and spectra that include these effects. We have performed phonon calculations for a number of different systems, including silicon, ZrH2, Carbazole and ZIF-8. Inelastic neutron scattering (INS) spectra can be rigorously calculated using the eigenvectors and eigenvalues from ab-initio phonon calculations; the quality of the theoretical predictions can be compared directly with experimental INS data. The reliability of the RMG code is verified by comparing the calculated results with other DFT codes and INS spectra measured by the VISION instrument at the Spallation Neutron Source at ORNL. We use the ALAMODE open-source software (sourceforge.net/projects/alamode) to extract the anharmonic force matrices from the dynamics and to obtain the phonon band structures. We present results comparing the AIMD-obtained phonon band structures to the experimental spectra.