Behavior Of Velocity Autocorrelation Function Research Articles

Atomistic tribological investigation of ultrathin layer of carbon disulfide between diamond surfaces

Using the method of classical molecular dynamics the tribological properties are studied of one molecular layer of liquid carbon disulfide, pressed by two atomically smooth diamond surfaces. We treat the models of elastic spheres for carbon disulfide molecules and absolutely rigid diamond plates. The found time series of atomic level friction force at the contact is heterogeneous. An increase of the external normal pressure leads to the transition of the film to a solidlike state. This is manifested in a decrease of the diffusion constant, in the behavior of velocity autocorrelation function of liquid molecules and average friction force.

Smoothed Dissipative Particle Dynamics package for LAMMPS

An isothermal implementation of Smoothed Dissipative Particle Dynamics (SDPD) for LAMMPS is presented. SDPD is useful for hydrodynamics simulations at mesoscale where the effect of thermal fluctuations are important, but a molecular dynamics simulation is prohibitively expensive. We have used this package to simulate diffusion of spherical colloids. The results (particularly the long-time behaviour of velocity autocorrelation function) are in agreement with theoretical models that take hydrodynamic interactions into account. Program summaryProgram Title: Smoothed Dissipative Particle HydrodynamicsProgram Files doi:http://dx.doi.org/10.17632/nhfp6nsr6n.1Licensing provisions: LGPLv3Programming language: C++Nature of problem: Multi-scale simulation of fluctuating hydrodynamics with consistent thermodynamics and hydrodynamic interactions in arbitrary geometry.Solution method: We have implemented an isothermal version of the Smoothed Dissipative Particle Hydrodynamics. SDPD is a multi-scale method as its input is only the resolution of particle grid used to model the fluid and macroscopic properties of fluid like viscosity and density (e.g there is no need to specify the strength of fluctuating force). SDPD respects the laws of thermodynamics and the Navier–Stokes equation by design.

Molecular dynamics of partially confined Lennard-Jones gases: Velocity autocorrelation function, mean squared displacement, and collective excitations

Particle motion and correlations in fluids within confined domains promise to provide challenges and opportunities for experimental and theoretical studies. We report molecular dynamics simulations of a Lennard-Jones gas mimicking argon under partial confinement for a wide range of densities at a temperature of 300 K. The isotropic behavior of velocity autocorrelation function (VACF) and mean squared displacement (MSD), seen in the bulk, breaks down due to partial confinement. A distinct trend emerges in the VACF-perpendicular and MSD-perpendicular, corresponding to the confined direction, while the trends in VACF-parallel and MSD-parallel, corresponding to the other two unconfined directions are seen to be unaffected by the confinement. VACF-perpendicular displays a minimum, at short timescales, that correlates with the separation between the reflective walls. The effect of partial confinement on MSD-perpendicular is seen to manifest as a transition from diffusive to subdiffusive motion with the transition time correlating with the minimum in the VACF-perpendicular. When compared to the trends shown by MSD and VACF in the bulk, the MSD-perpendicular exhibits subdiffusive behavior and the VACF-perpendicular features rapid decay, suggesting that confinement suppresses the role of thermal fluctuations significantly. Repetitive wall-mediated collisions are identified to give rise to the minima in VACF-perpendicular and in turn a characteristic frequency in its frequency spectrum. The strong linear relation between the minima in VACF-perpendicular and wall-spacing suggests the existence of collective motion propagating at the speed of sound. These numerical experiments can offer interesting possibilities in the study of confined motion with observable consequences.

Microscopic mechanisms of diffusion of higher alkanes

With the use of n-triacontane models as examples, abnormal characteristics of diffusion that manifest themselves during the application of the Einstein–Smoluchowski relationship and the asymptotic behavior of velocity autocorrelation function of the molecule-mass centers that is used to calculate the diffusion coefficient via the Green–Kubo formula are investigated. On the basis of the data of complementary approaches, the microscopic mechanisms of diffusion in higher alkanes are outlined. The applicability of the Stokes–Einstein relationship for the viscosity coefficient is demonstrated.

Erratum: “Long-Time Behavior of Velocity Autocorrelation Function for Interacting Particles in a Two-Dimensional Disordered System”

Erratum: “Long-Time Behavior of Velocity Autocorrelation Function for Interacting Particles in a Two-Dimensional Disordered System”

Long-Time Behavior of Velocity Autocorrelation Function for Interacting Particles in a Two-Dimensional Disordered System

The long-time behavior of the velocity autocorrelation function (VACF) is investigated by the molecular dynamics simulation of a two-dimensional system which has both a many-body interaction and a random potential. With strengthening the random potential by increasing the density of impurities, a crossover behavior of the VACF is observed from a positive tail, which is proportional to t^{-1}, to a negative tail, proportional to -t^{-2}. The latter tail exists even when the density of particles is the same order as the density of impurities. The behavior of the VACF in a nonequilibrium steady state is also studied. In the linear response regime the behavior is similar to that in the equilibrium state, whereas it changes drastically in the nonlinear response regime.