First-principles temperature-dependent phonons and elastic constants

Calculations of thermodynamic properties of materials from first-principles are critical for equation of state and materials strength modeling. ~Here we present the thermodynamic properties of a select set of metals based on density functional theory. In particular, we present elastic constants and lattice dynamics for body-centered cubic metals obtained from first-principles molecular dynamics and a self-consistent phonon approach. In order to calculate the thermodynamic properties, we make use of fluctuation formulas associated with the canonical ensemble form of \textit{ab initio} molecular dynamics (AIMD). This procedure is efficient and takes into account the anharmonic contributions to the equilibrium thermodynamic properties. In the self-consistent lattice dynamics approach, the phonon dispersions at finite temperature are determined from small displacements along normal modes associated with the chosen temperature.~ This method provides an efficient and accurate technique for phonon spectrum and finite-temperature acoustic sound speeds and elastic constants. We found that both methods provide consistent results for the temperature- and pressure-dependent elastic moduli. The AIMD include full anharmonicity but suffers from statistical errors of the order of 5{\%}. The self-consistent phonon method, on the other hand, has less statistical uncertainty but does not explicitly account for electron-phonon coupling. At ambient pressure, our calculations (both methods) agree quite well with experimental data.\\ \\This work performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.