Molecular Dynamics Simulations of Warm Dense Carbon

Abstract

We present classical and DFT-based molecular dynamics (MD) simulations of carbon in the warm dense matter regime (ρ= 3.7 g/cc, 0.86 eV < T < 8.62 eV [T < 100 eV for classical MD]). Two different classical interatomic potentials are used: 1. LCBOP, designed to simulate condensed (e.g. solid) phases of C, and 2. linearly screened Coulomb (Yukawa) potentials. It is shown that LCBOP over-predicts minima and maxima in the pair correlation functions of liquid-C in this regime when compared to the DFT-MD results. The screened Coulomb model, while under-correlating at low-T , seems to produce the correct qualitative features in the static ionic pair distributions at the highest-T . However, both approaches predict the decay in the ionic contribution of the specific heat as T −→ ∞ to be much slower than that predicted by a model based on DFT-MD. These differences in the MD-derived equations of state in warm dense regimes could have important consequences when using classical inter-ionic forces such as these in large-scale MD simulations aimed at studying, for instance, processes of relevance to inertial confinement fusion when C is used as an ablator material.

Isochoric, isobaric, and ultrafast conductivities of aluminum, lithium, and carbon in the warm dense matter regime

“…An early comparison of Car-Parrinello calculations for carbon with NPA was reported by Dharmawardana and Perrot in 1990 [42]. NPA successfully predicts the S(k) and g(r), inclusive of pre-peaks due to C-C bonding [20] as also obtained from DFT+MD simulations of WDM-carbon [18,19]. The NPA and Path Integral Monte Carlo g(r) [43] also agree closely [20].…”

Experimental Measurement of Self-Diffusion in a Strongly Coupled Plasma

“…This challenging fundamental problem is also one of the major limitations to our ability to accurately model equilibration, transport, and equations of state of dense laboratory and astrophysical plasmas [7,8], which impacts the design of inertialconfinement-fusion experiments [9,10], stellar chronometry based on white dwarf stars [11,12], and models of planet formation [13]. Molecular dynamics (MD) simulations have been the principal recourse for obtaining a microscopic understanding of short-time collision dynamics in this regime [14][15][16][17][18], but direct comparison of results with experiment has not been possible.…”

Theory of complex fluids in the warm dense matter regime and application to an unusual phase transition in liquid carbon

“…(a) Comparison of the S ( k ) of liquid carbon from the NPA + HNC with the DFT + MD simulation reported by Kraus et al at 0.86 eV and 3.7 g/cm 3 . (b) The corresponding neutral pseudo‐atom (NPA) C–C potential (in units of T ), the g ( r ) from the NPA, and the DFT + MD g ( r ) of Whitley et al…”

“…Hence, the use of T = 0 XC‐functionals and standard DFT + MD is justified. Kraus et al report the S ( k ) obtained from their simulations, while Whitley et al have published g ( r ) data for an overlapping regime. We use these results and the PIMC predictions to benchmark the NPA method against DFT + MD as well as PIMC methods.…”

“…The inversion of a g ( r ) to a potential requires a valid first‐principles form for V ii ( r ) and an accurate bridge function, but it does not call for dynamic structure data , . One may also proceed as in Whitley et al, who derived a C–C potential at 0.86 eV by force matching to DFT + MD (their Figure a). Their extracted potential is in good agreement with the NPA potential, Figure b, but the Friedel oscillations are poorly captured by force‐matching methods.…”