A first order phase transition is found in liquid carbon using atomistic simulation methods and Brenner's bond order potential. The phase line is terminated by a critical point at 8801 K and 10.56 GPa and by a triple point on the graphite melting line at 5133 K and 1.88 GPa. The phase change is associated with density and structural changes. The low-density liquid is predominantly sp bonded with little sp 3 character. The high-density liquid is mostly sp 3 bonded with little sp character. This is the first nonempirical evidence of a liquid-liquid transition between thermodynamically stable fluids.[ S0031-9007(99) . This careful work on the melting of graphite by Togaya suggests that the slope of pressure-temperature ͑P-T ͒ melting line is discontinuous at the temperature maximum, and hence at least three stable phases coexist at this point. The most likely conclusion is that this point is a triple point and the carbon phase diagram exhibits a LLPT.Though somewhat exotic, LLPT's have been suggested in liquid S, Ga, Se, Te, I 2 , Cs, and Bi [4]. Recent experiments [5] and molecular dynamics (MD) calculations [6] have shown evidence of a first order transition between metastable phases of supercooled water.The purpose of this Letter is to report the findings of a MD simulation of liquid carbon showing clear evidence of a LLPT. The LLPT line in carbon is mapped out, including the critical point at high T and the triple point at low T . The low-density liquid (LDL) phase has different local structure than the high-density liquid (HDL) phase. An order parameter sensitive to this local order change is defined and examined as a function of T and P. This is the first time that atomic simulations have directly observed a first order LLPT between two distinct thermodynamically stable liquid phases in carbon or any other material.To describe the carbon-carbon interaction, the hydrocarbon potential model developed by Brenner [7] is used. The Brenner potential falls in a class of empirical potentials known as bond-order potentials having the simple formwhere f R and f A are exponential functions representing the repulsive and attractive terms, respectively, in the bond energy. Although a sum over bond energies (͕bonds͖ is the set of all bonds), this is not a pair potential. The so-called bond-order factor b ij is a many-body factor and depends on bond angles, torsional angles, bond lengths, and atomic coordination in the vicinity of the bond. This many-body nature of b ij allows bond energy to depend on the local environment in such a way so as to produce the correct geometry and energy for many different carbon structures including graphite and diamond.The simulations are performed mostly on 512 particle systems using a constant (cubic) volume ͑V ͒ and temperature MD. A number of calculations were done with 4096 particles to check for finite-size effects. The equations of motion are integrated using a velocity-Verlet algorithm [8] with temperature controlled via the Nóse-Hoover thermostat [9]. To control integration errors,...

The temperature equilibration rate between electrons and protons in dense hydrogen has been calculated with molecular dynamics simulations for temperatures between 10 and 600eV and densities between 10;{20}cm;{-3}to10;{24}cm;{-3} . Careful attention has been devoted to convergence of the simulations, including the role of semiclassical potentials. We find that for Coulomb logarithms L greater, similar1 , a model by Gericke-Murillo-Schlanges (GMS) [D. O. Gericke, Phys. Rev. E 65, 036418 (2002)] based on a T -matrix method and the approach by Brown-Preston-Singleton [L. S. Brown, Phys. Rep. 410, 237 (2005)] agrees with the simulation data to within the error bars of the simulation. For smaller Coulomb logarithms, the GMS model is consistent with the simulation results. Landau-Spitzer models are consistent with the simulation data for L>4 .

Molecular dynamics is used to model the friction between two ordered monolayers of alkane chains six units long bound at their ends to two rigid substrates. Depending on the interfacial interaction strength, energy dissipation occurs by a discontinuous "plucking" mechanism and a continuous "viscous" mechanism. The plucking mechanism follows a simple thermal activation model, while for the viscous mechanism, friction force is enhanced at the rotator transition temperature of the films.PACS numbers: 68.35.Gy, 46.30.Pa, 68.35.Md, 81.40.Pq Interfacial friction and wear between two sliding surfaces can be minimized by binding to each surface films which interact with each other only weakly. We have used molecular dynamics (MD) to study this boundary lubrication process between films of close-packed hydrocarbon chains six units long. Two types of energy dissipation are found: a "plucking" mechanism associated with sudden release of shear strain, and a more continuous "viscous" behavior arising from continuous collisions of atoms of opposite films. The influence of interaction strength, temperature, and velocity on these mechanisms is described. The surface force apparatus [1-4], the force microscope [5-9], and a new crystal oscillator technique [10] have recently been used to understand sliding friction at the atomic level. MD has been used to simulate friction of a variety of types of materials [11], and the dynamics of organized alkane chain monolayers systems have been modeled [12][13][14][15], but MD has not been applied previously to friction between organic monolayers.We model the films by a 6 x 6 array of alkane chains of six monomers at rigid bond lengths (Fig. 1), extended with periodic boundary conditions parallel to the substrates [16]. The CH3 and CH2 groups are treated as single spherical united atoms. For the eff'ective interaction potential between nearby atoms on the same chain, we sum the bond-bending potential of Ref. [17] with the torsion potential of Ref. [18]. For pairs of atoms on the same chain more distant than third nearest neighbors and for all interactions between chains, we use the LennardJones potential (7LJ('') =4e[(cj/r)*^ -(cr/r)^], where 20.43 A FIG. 1. Top and side view of the computational cell, showing the initial configuration and sliding direction. The JC axis is the sliding direction. (T = 3.965 A and r is the atom pair separation. The interaction strength s is eo^'lOA K [19], except for interactions between atoms across the interface, where it is designated s\. We vary s\ to simulate changing the end group and to elucidate frictional mechanisms. For smoothing, the nonbonded interactions are truncated at r =rc =2.25o-by multiplying ^LJC'*) by [1 -{r/rc)^] ^ for r < r^ and by 0 otherwise. For all atoms, the total potential includes a substrate interaction (5.3x10^ K)/z^^ -(2.1x10"* K)/z^ depending on the distance z (in A) of the atom from the substrate. An isotropic harmonic potential, with force constant 50 K/A^, pins one of the end atoms of each chain to a 2D array with spacin...

We study the problem of electron-ion temperature equilibration in plasmas. We consider pure H at various densities and temperatures, and Ar-doped H at temperatures high enough so that the Ar is fully ionized. Two theoretical approaches are used: classical molecular dynamics (MD) with statistical 2-body potentials, and a generalized Lenard-Balescu (GLB) theory capable of treating multi-component weakly-coupled plasmas. The GLB is used in two modes: 1) with the quantum dielectric response in the random-phase approximation (RPA) together with the pure Coulomb interaction, and 2) with the classical (h −→ 0) dielectric response (both with and without local-field corrections) together with the statistical potentials. We find that the MD results are described very well by classical GLB including the statistical potentials and without local-field corrections (RPA only); worse agreement is found when static local-field effects are included, in contradiction to the classical pure-Coulomb case with like charges. The results of the various approaches are all in excellent agreement with pure-Coulomb quantum GLB when the temperature is high enough.In addition, we show that classical calculations with statistical potentials derived from the exact quantum 2-body density matrix produce results in far better agreement with pure-Coulomb quantum GLB than classical calculations performed with older existing statistical potentials.

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RAS is a signaling protein associated with the cell membrane that is mutated in up to 30% of human cancers. RAS signaling has been proposed to be regulated by dynamic heterogeneity of the cell membrane. Investigating such a mechanism requires near-atomistic detail at macroscopic temporal and spatial scales, which is not possible with conventional computational or experimental techniques. We demonstrate here a multiscale simulation infrastructure that uses machine learning to create a scale-bridging ensemble of over 100,000 simulations of active wild-type KRAS on a complex, asymmetric membrane. Initialized and validated with experimental data (including a new structure of active wild-type KRAS), these simulations represent a substantial advance in the ability to characterize RAS-membrane biology. We report distinctive patterns of local lipid composition that correlate with interfacially promiscuous RAS multimerization. These lipid fingerprints are coupled to RAS dynamics, predicted to influence effector binding, and therefore may be a mechanism for regulating cell signaling cascades.

Estimates for the displacement of the phase equilibrium lines for small carbon particles containing from several hundred to several tens of thousands of atoms are made, and an error analysis of the uncertainties in these estimates is derived and evaluated using available experimental data. Hugoniot calculations for methane, benzene, polyethylene, and polybutene, based on a carbon particle surface energy adjusted equation of state, are in better agreement with shock pressure-volume and temperature data than those obtained with a bulk carbon equation of state. The results suggest that carbon particles, of order 103–104 atoms, can exist in the liquid state at lower temperatures than bulk carbon.

It has been recently suggested that elemental carbon may be a promising candidate to exhibit a liquid-liquid phase transition (LLPT). We report the results of first-principles molecular dynamics simulations showing no evidence of LLPT in carbon, in the same temperature and pressure range where such a transition was found using empirical calculations. Our simulations indicate a continuous evolution from a primarily sp-bonded liquid to an sp(2)-like and an sp(3)-like fluid, as a function of pressure, above the graphite melting line. The discrepancy between quantum and classical simulations is attributed to the inability of empirical potentials to describe complex electronic effects in condensed carbon phases.