Constant-force approach to discontinuous potentials

Pseudo hard-sphere viscosities from equilibrium Molecular Dynamics

Transport coefficients like shear, bulk and longitudinal viscosities are sensitive to the intermolecular interaction potential and finite size effects when are numerically determined. For the hard-sphere (HS) fluid, such transport properties are determined almost exclusively with computer simulations. However, their systematic determination and analysis throughout shear stress correlation functions and the Green-Kubo formalism can not be done due to discontinuous nature of the interaction potential. Here, we use the pseudo hard-sphere (PHS) potential to determine pressure correlation functions as a function of volume fraction in order to compute mentioned viscosities. Simulation results are compared to available event-driven molecular dynamics of the HS fluid and also used to propose empirical corrections for the Chapman-Enskog zero density limit of shear viscosity. Moreover, we show that PHS potential is a reliable representation of the HS fluid and can be used to compute transport coefficients. The molecular simulation results of the present work are valuable for further exploration of HS-type fluids or extend the approach to compute transport properties of hard-colloid suspensions.

The generalized continuous multiple step (GCMS) potential: model systems and benchmarks

The generalized continuous multiple step (GCMS) potential is presented in this work. Its flexible form allows forrepulsiveand/orattractivecontributions to be encoded through adjustable energy and length scales. The GCMS interaction provides a continuous representation of square-well, square-shoulder potentials and their variants for implementation in computer simulations. A continuous and differentiable energy representation is required to derive forces in conventional simulation algorithms. Molecular dynamics simulations are of particular interest when considering the dynamic properties of a system. The GCMS potential can mimic other interactions with a judicious choice of parameters due to the versatile sigmoid form. In this study, our benchmarks for the GCMS representation include triangular, Yukawa, Franzese, and Lennard-Jones potentials. Comparisons made with published data on volumetric phase diagrams, liquid structure, and diffusivity from model systems are in excellent agreement.

Soft representation of the square-well and square-shoulder potentials to be used in Brownian and molecular dynamics simulations

The discrete hard-sphere (HS), square-well (SW), and square-shoulder (SS) potentials have become the battle horse of molecular and complex fluids because they contain the basic elements to describe the thermodynamic, structural, and transport properties of both types of fluids. The mathematical simplicity of these discrete potentials allows us to obtain some analytical results despite the nature and complexity of the modeled systems. However, the divergent forces arising at the potential discontinuities may lead to severe issues when discrete potentials are used in computer simulations with uniform time steps. One of the few routes to avoid these technical problems is to replace the discrete potentials with continuous and differentiable forms built under strict physical criteria to capture the correct phenomenology. The match of the second virial coefficient between the discrete and the soft potentials has recently been successfully used to construct a continuous representation that mimics some physical properties of HSs (Báezet al2018J. Chem. Phys.149164907). In this paper, we report an extension of this idea to construct soft representations of the discrete SW and SS potentials. We assess the accuracy of the resulting soft potential by studying structural and thermodynamic properties of the modeled systems by using extensive Brownian and molecular dynamics computer simulations. Besides, Monte Carlo results for the original discrete potentials are used as benchmark. We have also implemented the discrete interaction models and their soft counterparts within the integral equations theory of liquids, finding that the most widely used approximations predict almost identical results for both potentials.

Self-diffusion coefficient of the square-well fluid from molecular dynamics simulations within the constant force approach

We present a systematic study of the self-diffusion coefficient for a fluid of particles interacting via the square-well pair potential by means of molecular dynamics simulations in the canonical (N, V, T) ensemble. The discrete nature of the interaction potential is modeled by the constant force approximation, and the self-diffusion coefficient is determined for several fluid densities at supercritical thermodynamic states. The dependence of the self-diffusion coefficient on the potential range λ is analyzed in the range of 1.1 ≤ λ ≤ 1.5. The obtained simulation results are in agreement with the self-diffusion coefficient predicted by the Enskog method. Additionally, we show that the diffusion coefficient is very sensitive to the potential range λ. Our results for the self-diffusion coefficient times density extrapolate well to the values in the zero-density limit obtained from the Chapman-Enskog theory for dilute gases. The constant force approximation used in this work to model the discrete pair potentials has shown to be an excellent scheme to compute the transport properties of square-well fluids using molecular dynamics simulations. Finally, the simulation results presented here are useful for improving theoretical approaches, such as the Enskog method.

Conformation change of an isotactic poly (N-isopropylacrylamide) membrane: Molecular dynamics

In this work, isotactic Poly (N-Isopropylacrylamide)-PNIPAM-in neat water and in electrolyte solutions is studied by means of molecular dynamics simulations. This is done for an infinitely diluted oligomer and for an assembly of several PNIPAM chains arranged into a planar membrane configuration with a core-shell morphology. We employed two different force fields, AMBER (assisted model building with energy refinement) and OPLS-AA (all atom - optimized potentials for liquid simulations) in combination with extended simple point charge water. Despite the more water insoluble character of isotactic oligomers, our results support the existence of a coil to globule transition for the isolated 30-mer. This may imply the existence of an oligomer rich phase of coil-like structures in equilibrium with a water rich phase for temperatures close but below the coil to globule transition temperature, T_Θ. However, the obtained coil structure is much more compact than that corresponding to the syndiotactic chain. Our estimations of T_Θ are (308±5) K and (303±5) K for AMBER and OPLS-AA, respectively. The membrane configuration allows one to include chain-chain interactions, to follow density profiles of water, polymer, and solutes, and accessing the membrane-water interface tension. Results show gradual shrinking and swelling of the membrane by switching temperature above and below T_Θ, as well as the increase and decrease of the membrane-water interface tension. Finally, concentration profiles for 1M NaCl and 1M NaI electrolytes are shown, depicting a strong salting-out effect for NaCl and a much lighter effect for NaI, in good qualitative agreement with experiments.

The constant force continuous molecular dynamics for potentials with multiple discontinuities

In this work, we present an extension of the constant force approach [P. Orea and G. Odriozola, J. Chem. Phys. 138, 214105 (2013)] to the case of potentials with multiple discontinuities. To illustrate the method, we selected the square well potential of range λ=1.5 that exhibits two discontinuities. Square well single phase properties, vapor-liquid phase diagram, and surface tension were calculated and compared with available simulation data. Besides, we analyzed the internal energies of a square well plus a square shoulder potential having three discontinuities. For both potentials, a good agreement has been found when compared with results of other simulation techniques (discontinuous molecular dynamics and Monte Carlo methods). This extension can be easily implemented to more general and efficient continuous molecular dynamics packages (HOOMD, GROMACS, NAMD, etc.).

Thermodynamic properties of triangle-well fluids in two dimensions: MC and MD simulations

With the aim of providing complementary data of the thermodynamics properties of the triangular well potential, the vapor/liquid phase diagrams for such potential with different interaction ranges were calculated in two dimensions by Monte Carlo and molecular dynamics simulations; also, the vapor/liquid interfacial tension was calculated. As reported for other interaction potentials, it was observed that the reduction of the dimensionality makes the phase diagram to shrink. Finally, with the aid of reported data for the same potential in three dimensions, it was observed that this potential does not follow the principle of corresponding states.