Session J2: Potentials for Large-Scale Shock Simulations

Interatomic Potentials for Large-Scale Simulations of High-Pressure, High-Temperature Phenomena

The use of large-scale atomistic simulations in the study of high-compression, high strain-rate phenomena has dramatically increased in the last five years. Most of this type of simulations utilize classical empirical or semi-empirical potentials to describe the inter-atomic interactions. The regime of validity of these potentials is however often limited to a narrow region of the pressure-temperature phase diagram. In constructing interatomic potentials for high-pressure high-temperature applications, it is desirable to obtain a high degree of transferability without resorting to fitting everywhere in phase space. We will review two popular cluster functional models, the embedded-atom-method (EAM) and the modified embedded-atom-method (MEAM). The EAM provides a very simple description of the many-body cohesion in metals and MEAM is an improvement of EAM which includes the effect of angular bonding. We outline inherent limitations of these models and present a systematic approach to improving their transferability and predictive accuracy.   

Molecular Dynamics Simulation of Thermodynamic and Mechanical Properties of Be and Mg

At ambient conditions Beryllium and Magnesium have HCP with the non-ideal c/a ration. This circumstance does not allow using the simple, spherically symmetric potential in MD simulations. The Modified Embedded Atom Model (MEAM) accounting for local environment and including angular forces has been applied to MD simulation of thermodynamic and mechanical properties of Be and Mg. Parameters of the MEAM have been fitted to available data on Be and Mg bulk and microscopic properties at ambient conditions using the theory of MEAM by M. Baskes. Bulk and shear moduli of monocrystalline materials have been calculated as functions of pressure and temperature. Ab-initio (DFT) calculations have also been carried out to obtain pressure dependence of elastic constants. Result of MD and ab-initio calculations have been averaged to obtain the corresponding values for polycrystalline materials and have been compared with available experimental data and phenomenological models.   

Transforming graphite to diamond: An \textit{ab initio} molecular dynamics study of graphite under shock compression

We present an extremely large scale \textit{ab initio} calculation of the transformation of graphite to diamond under shock compression utilizing Car-Parrinello Molecular Dynamics (CPMD) in conjunction with the Multi-Scale Shock Method (MSSM). Our results indicate that the transition from graphite to diamond is Martensitic, in agreement with experimental observations. We find that a shock of 12 km/s forms a short-lived layered diamond phase which eventually relaxes to a cubic diamond state. Moreover, access to the electronic structure allows the computation the x-ray absorption spectra (XAS) to characterize the final states. The XAS spectra and wide angle x-ray scattering spectra (WAXS) confirm the presence of a cubic diamond final state.   

Discussion Session: The Quality of Interaction Potentials in Molecular-Dynamics Calculations Under Extreme Conditions

How good are the interaction potentials used in classical molecular-dynamics (MD) simulations at high pressures and temperatures? A variety of semi-emprical functional forms have been used in large-scale MD simulations of shockwave phenomena, for example. These potential functions make possible more efficient large-scale simulations of extreme conditions than will (at least for some time to come) be possible by ab-initio quantum-mechanical (QM) MD. The potential-function parameters have traditionally been fitted to experimental properties at low pressures and temperatures, with little information contributed from experiments under extreme conditions. As a result, one can legitimately worry about the quality of MD simulations in such regimes. Modern computational capabilities have enabled the use of many more high-quality QM calculations for high-pressure, zero-temperature properties, which could be of considerable use in extending the phase space for fitting empirical parameters. However, one can ask whether these QM calculations are sufficiently accurate to aid the fitting process, and even more fundamentally, whether the current set of semi-empirical potentials even have the right functional forms. In addition, the effects of high temperatures on the fundamental physics (or chemistry) of the potentials used is almost entirely unexplored territory. Small-scale QM MD could contribute a great deal to this topic, if one were convinced of the quality of those simulations. It is hoped that the members of the audience who have had experience in using any of these semi-empirical potentials, or in generating QM data for fitting their parameters, will share some comments. The time is ripe for new paradigms in the exchange of information between QM and classical MD.