Elastic constants of sodium from molecular dynamics

We have performed molecular-dynamics calculations of the adiabatic elastic constants of sodium at three different temperatures, T=198, 299, and 349 K. Our method uses fluctuation formulas appropriate for the microcanonical ensemble which contain the elastic constants. In the simulation we have used a first-principles potential to model the interaction between the sodium atoms. The results, including the shear modulus C44, show good agreement with experiment at all three temperatures. We have analyzed the contributions to the elastic constants from different types of terms appearing in the fluctuation formula and compared these contributions to other model-potential calculations. The volume dependence in the potential has considerable effect on the values of elastic constants. In comparison to some earlier calculations which employed pair potentials with no volume dependence, the fluctuation contributions to elastic constants C11 and C44 are noticeably large (20% of the value of the elastic constants in some cases). We find that the elastic constants do not change by much for the different potential-cutoff ranges employed: 17.85, 23.27, and 27.70 bohrs.