Simulation of molten NaI including polarization effects

First principles molecular dynamics of molten NaI: Structure, self-diffusion, polarization effects, and charge transfer

The structure and self-diffusion of NaI and NaCl at temperatures close to their melting points are studied by first principles Hellmann-Feynman molecular dynamics (HFMD). The results are compared with classical MD using rigid-ion (RI) and shell-model (ShM) interionic potentials. HFMD for NaCl was reported before at a higher temperature [N. Galamba and B. J. Costa Cabral, J. Chem. Phys. 126, 124502 (2007)]. The main differences between the structures predicted by HFMD and RI MD for NaI concern the cation-cation and the anion-cation pair correlation functions. A ShM which allows only for the polarization of I- reproduces the main features of the HFMD structure of NaI. The inclusion of polarization effects for both ionic species leads to a more structured ionic liquid, although a good agreement with HFMD is also observed. HFMD Green-Kubo self-diffusion coefficients are larger than those obtained from RI and ShM simulations. A qualitative study of charge transfer in molten NaI and NaCl was also carried out with the Hirshfeld charge partitioning method. Charge transfer in molten NaI is comparable to that in NaCl, and results for NaCl at two temperatures support the view that the magnitude of charge transfer is weakly state dependent for ionic systems. Finally, Hirshfeld charge distributions indicate that differences between RI and HFMD results are mainly related to polarization effects, while the influence of charge transfer fluctuations is minimal for these systems.

Shear viscosity of molten alkali halides from equilibrium and nonequilibrium molecular-dynamics simulations

The shear viscosity of molten NaCl and KCl was calculated through equilibrium (EMD) and nonequilibrium molecular-dynamics (NEMD) simulations in the canonical (N,V,T) ensemble. Two rigid-ion potentials were investigated, namely, the Born-Mayer-Huggins-Tosi-Fumi potential and the Michielsen-Woerlee-Graaf-Ketelaar potential with the parameters proposed by Ladd. The NEMD simulations were performed using the SLLOD equations of motion [D. J. Evans and G. P. Morriss, Phys. Rev. A 30, 1528 (1984)] with a Gaussian isokinetic thermostat and the results are compared with those obtained from Green-Kubo EMD (N,V,T) simulations and experimental shear viscosity data. The NEMD zero strain rate shear viscosity, eta(0), was obtained by fitting a simplified Carreau-type equation and by application of mode-coupling theory, i.e., a eta-gamma(1/2) linear relationship. The values obtained from the first method are found to be significantly lower than those predicted by the second. The agreement between the EMD and NEMD results with experimental data is satisfactory for the two potentials investigated. The ion-ion radial distribution functions obtained with the two rigid-ion potentials for both molten salts are discussed in terms of the differences between the two models.