N. V. Gribova

Waterlike thermodynamic anomalies in a repulsive-shoulder potential system

We report a computer-simulation study of the equilibrium phase diagram of a three-dimensional system of particles with a repulsive-shoulder potential. The phase diagram was obtained using free-energy calculations. At low temperatures, we observe a number of distinct crystal phases. We show that at certain values of the potential parameters the system exhibits the waterlike thermodynamic anomalies: a density anomaly and a diffusion anomaly. The anomalies disappear with increasing the repulsive step width: more precisely, their locations move to the region where the crystalline phase is stable.

Breakdown of excess entropy scaling for systems with thermodynamic anomalies

This paper presents a simulation study of the applicability of the Rosenfeld entropy scaling to the systems which cannot be approximated by the effective hard spheres. Three systems are studied: the Herzian spheres, the Gauss core model, and a soft repulsive shoulder potential. These systems demonstrate diffusion anomalies at low temperatures: the diffusion coefficient increases with increasing density or pressure. It is shown that for the first two systems belonging to a class of bounded potentials, the Rosenfeld scaling formula is valid only in the infinite-temperature limit where there are no anomalies. For the soft repulsive shoulder potential, the scaling formula is valid already at sufficiently low temperatures, however, out of the anomaly range.

How close to two dimensions does a Lennard-Jones system need to be to produce a hexatic phase?

We report on a computer simulation study of a Lennard-Jones liquid confined in a narrow slit pore with tunable attractive walls. In order to investigate how freezing in this system occurs, we perform an analysis using different order parameters. Although some of the parameters indicate that the system goes through a hexatic phase, other parameters do not. This shows that to be certain whether a system of a finite particle number has a hexatic phase, one needs to study not only a large system, but also several order parameters to check all necessary properties. We find that the Binder cumulant is the most reliable one to prove the existence of a hexatic phase. We observe an intermediate hexatic phase only in a monolayer of particles confined such that the fluctuations in the positions perpendicular to the walls are less than 0.15 particle diameters, i.e., if the system is practically perfectly 2D.

Complex Phase Behavior of Systems with Negative Curvature Potentials

The comprehensive computer simulation study of the phase diagram of the repulsive step potential system in three dimensions is represented. We show that the system with a simple purely repulsive isotropic potential demonstrates a number of unusual features. The maxima and minima on the melting curve are found for some regions of potential parameters. It is shown that the phase diagram in ρ-T plane includes two isostructural crystalline parts separated by the disordered phase which is amorphous at low enough temperatures. Phase diagram in the (P-T) plane shows that the transition to the amorphous state occurs approximately along the extrapolated spinodals. Structural FCC-BCC phase transition is found at high densities.

Effective potentials between gold nano crystals – functional dependence on temperature

A method is presented that allows to combine the effective potential between two nano crystals (NC), the potential of mean force (PMF), as obtained from all-atomistic molecular dynamics simulations with perturbation theory. In this way, a functional dependence of the PMF on temperature is derived, which enables the prediction of the PMF in a wide temperature range. We applied the method to systems of capped gold NCs of different size. They show very good agreement with data from atomistic simulations.

Three-body effects in triplets of capped gold nanocrystals

ABSTRACT Three-body interactions constitute an important part of the effective potential between nanocrystals (NCs). In this study, molecular dynamics simulations are conducted on gold NCs capped with alkyl thiol ligands in vacuum. Over the course of a simulation performed in two- and three-body systems, we measure the forces acting on the cores of the NCs. These forces are then used to calculate the two- and three-body potentials of mean force (PMF). The influence of the ligand length, the size of the core and the temperature are studied. We find that three-body effects are mainly repulsive. Longer ligand lengths and bigger core sizes further increase the strength of repulsion. According to our simulation data, the three-body contribution is independent of the temperature. Furthermore, we propose an empirical model of the three-body contribution based on the repulsive part of the two-body PMF.

Different ways of looking at the force between two nanocrystals

The potential of mean force (PMF) between two nanocrystals (NCs) represents an effective interaction potential that is essential when explaining the assembly of NCs to superstructures. For a given temperature, the PMF is obtained best from molecular dynamics simulations. Based on a density functional approach, this study proposes three methods of predicting the PMF for any given temperature based on a single molecular dynamics simulation for one temperature. The three methods construct the PMF by considering the ligands as an ideal gas, as hard-sphere chains, or as Lennard-Jones interaction sites. To apply this methodology, the density of the interaction centers must be extracted from the simulation data. For the ideal gas model, a straightforward sampling procedure with a fixed lattice in space leads to free energies that are too large in order to consistently explain the simulation data for different temperatures. Naive sampling does not account for the small momenta added to the NCs when coupled to a thermostat. A method is proposed that corrects for the unphysical steps during the simulation. The ideal gas contribution computed for the corrected density is significantly smaller than the one obtained from naive sampling and can thus explain the temperature dependence of the PMF correctly. For the hard-sphere chain model, where a weighted density is used, the correction of the particle density is not essential. However, the PMF calculated based on the corrected density confirms our approach. All three models predict PMF curves in very good agreement with simulation results, but they differ in the number of input parameters and the computational effort. Based on the modeling results, we predict the existence of an additional attractive force at small distances of the NCs - a depletion force.