In this work, an extension is proposed to the standard iterative Boltzmann inversion (IBI) method used to derive coarse-grained potentials. It is shown that the inclusion of target data from multiple states yields a less state-dependent potential, and is thus better suited to simulate systems over a range of thermodynamic states than the standard IBI method. The inclusion of target data from multiple states forces the algorithm to sample regions of potential phase space that match the radial distribution function at multiple state points, thus producing a derived potential that is more representative of the underlying interactions. It is shown that the algorithm is able to converge to the true potential for a system where the underlying potential is known. It is also shown that potentials derived via the proposed method better predict the behavior of n-alkane chains than those derived via the standard IBI method. Additionally, through the examination of alkane monolayers, it is shown that the relative weight given to each state in the fitting procedure can impact bulk system properties, allowing the potentials to be further tuned in order to match the properties of reference atomistic and/or experimental systems.

Direct molecular dynamics simulations of the liquid-liquid interface of water–n-alkane and water-methanol–n-alkane systems have been performed in order to study the interfacial properties of these systems. The simulations were carried out using the NERD revised force field of Nath et al. for the n-alkanes, the simple point charge extended model for water, and the optimized potential for liquid simulations model for methanol. In order to validate the model employed in this work for the n-alkanes we calculated the coexisting densities, surface tension, and thickness of the interface for pure n-pentane. For all the systems studied the interfacial tension and thickness were calculated at 298.15 K. Our results show that, by adjusting the number of molecules to reproduce the liquid densities in the direct simulation method of the liquid-liquid interface in multicomponent systems, we are able to reproduce available experimental data for interfacial tension. The interfacial thickness is underpredicted and a constant negative deviation of $\sim 2.5\mathrm{\AA }$ from the experimental data is usually observed. We find that methanol acts like surfactant when it is added to the water–n-alkane mixtures, reducing the interfacial tension of the liquid-liquid ternary system. The interfacial tension results agree quantitatively well for the range of concentrations of methanol studied.

Coarse-grained (CG) models have proven to be very effective tools in the study of phenomena or systems involving large time- and length-scales. By decreasing the degrees of freedom in the system and using softer interactions than seen in atomistic models, larger time steps can be used and much longer simulation times can be studied. CG simulations are widely used to study systems of biological importance that are beyond the reach of atomistic simulation, necessitating a computationally efficient and accurate CG model for water. In this review, we discuss the methods used for developing CG water models and the relative advantages and disadvantages of the resulting models. In general, CG water models differ with regard to how many waters each CG group or bead represents, whether analytical or tabular potentials have been used to describe the interactions, and how the model incorporates electrostatic interactions. How the models are parameterised, which typically depends on their application, is also discussed.

The statistical associating fluid theory for potentials of variable attractive range (SAFT-VR) has been used in the calculation of the phase equilibria for long-chain n-alkanes and their mixtures. We treat the molecules as chains formed from united-atom hard-sphere segments, with a square-well potential of variable attractive range to describe the dispersive forces. A empirical relationship derived in earlier work is used to determine the number of such segments in relation to the carbon number for each member of the homologous series. Simple linear relationships between the potential model parameters and molecular weight have been determined enabling predictions of the fluid phase equilibria of heavier n-alkane molecules for which no experimental data are available. This is illustrated by a comparison with simulation data from the literature for the coexisting densities of n-octatetracontane (C₄₈H₉₈). Additionally we have examined binary mixtures of methane+n-hexadecane (C₁₆H₃₄) and hexane+n-tetradecane (C₁₄H₃₀) with the SAFT-VR approach, using simple Lorentz–Berthelot combining rules to determine the unlike interaction parameters. This study will allow an extension of the SAFT-VR approach to polymeric systems.