Simple liquids are traditionally defined as many-body systems of classical particles interacting via radially symmetric pair potentials. We suggest that a simple liquid should be defined instead by the property of having strong correlation between virial and potential energy equilibrium fluctuations in the N V T ensemble. There is considerable overlap between the two definitions, but also some notable differences. For instance, in the new definition simplicity is not a property of the intermolecular potential because a liquid is usually only strongly correlating in part of its phase diagram. Moreover, according to the new definition not all simple liquids are atomic (i.e., with radially symmetric pair potentials) and not all atomic liquids are simple. The main part of the paper motivates the new definition of liquid simplicity by presenting evidence that a liquid is strongly correlating if and only if its intermolecular interactions may be ignored beyond the first coordination shell (FCS). This is demonstrated by N V T simulations of structure and dynamics of atomic and molecular model liquids with a shifted-forces cutoff placed at the first minimum of the radial distribution function. The liquids studied are: inverse power-law systems (r −n pair potentials with n = 18, 6, 4), Lennard-Jones (LJ) models (the standard LJ model, two generalized Kob-Andersen binary LJ mixtures, the Wahnström binary LJ mixture), the Buckingham model, the Dzugutov model, the LJ Gaussian model, the Gaussian core model, the Hansen-McDonald molten salt model, the Lewis-Wahnström OTP model, the asymmetric dumbbell model, and the rigid SPC/E water model. The final part of the paper summarizes most known properties of strongly correlating liquids, showing that these are simpler than liquids in general. Simple liquids as defined here may be characterized (1) chemically by the fact that the liquid's properties are fully determined by interactions from the molecules within the FCS, (2) physically by the fact that there are isomorphs in the phase diagram, i.e., curves along which several properties like excess entropy, structure, and dynamics, are invariant in reduced units, and (3) mathematically by the fact that the reduced-coordinate constant-potential energy hypersurfaces define a one-parameter family of compact Riemannian manifolds. No proof is given that the chemical characterization follows from the strong correlation property, but it is shown to be consistent with the existence of isomorphs in strongly correlating liquids' phase diagram. Finally, we note that the FCS characterization of simple liquids calls into question the basis for standard perturbation theory, according to which the repulsive and attractive forces play fundamentally different roles for the physics of liquids.

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We show that for any liquid or solid with strong correlation between its NVT virial and potential-energy equilibrium fluctuations, the temperature is a product of a function of excess entropy per particle and a function of density, T = f(s)h(ρ). This implies that (1) the system's isomorphs (curves in the phase diagram of invariant structure and dynamics) are described by h(ρ)/T = Const., (2) the density-scaling exponent is a function of density only, and (3) a Grüneisen-type equation of state applies for the configurational degrees of freedom. For strongly correlating atomic systems one has h(ρ) = ∑nCnρn/3 in which the only non-zero terms are those appearing in the pair potential expanded as v(r) = ∑nvnr−n. Molecular dynamics simulations of Lennard-Jones type systems confirm the theory.

The contact angles of liquids and droplets of Lennard-Jones particles on a solid surface are determined by molecular dynamics simulations. The simulations show that the angles of contact are established within the first fluid layer. The droplets are not spherical segment-shaped. For an attractive surface corresponding to a small contact angle, the observed contact angles disagree with the corresponding angles obtained for macroscopic systems and using Young's equation and its extension for droplets with line tension.

Isomorphs are curves in the phase diagram along which a number of static and dynamic quantities are invariant in reduced units (Gnan, N.; et al. J. Chem. Phys.2009, 131, 234504). A liquid has good isomorphs if and only if it is strongly correlating, i.e., if the equilibrium virial/potential energy fluctuations are more than 90% correlated in the NVT ensemble. Isomorphs were previously discussed with a focus on atomic systems. This paper generalizes isomorphs to liquids composed of rigid molecules and study the isomorphs of systems of small rigid molecules: the asymmetric dumbbell model, a symmetric inverse power-law dumbbell, and the Lewis–Wahnström o-terphenyl (OTP) model. For all model systems, the following quantities are found to a good approximation to be invariant along an isomorph: the isochoric heat capacity, the excess entropy, the reduced molecular center-of-mass self-part of the intermediate scattering function, and the reduced molecular center-of-mass radial distribution function. In agreement with theory, we also find that an instantaneous change of temperature and density from an equilibrated state point to an isomorphic state point leads to no relaxation. The isomorphs of the Lewis–Wahnström OTP model were found to be more approximative than those of the asymmetric dumbbell model; this is consistent with the OTP model being less strongly correlating. The asymmetric dumbbell and Lewis–Wahnström OTP models each have a “master isomorph”; i.e., the isomorphs have identical shape in the virial/potential energy phase diagram.

The properties of nanoconfined fluids can be strikingly different from those of bulk liquids. A basic unanswered question is whether the equilibrium and dynamic consequences of confinement are related to each other in a simple way. We study this question by simulation of a liquid comprising asymmetric dumbbell-shaped molecules, which can be deeply supercooled without crystallizing. We find that the dimensionless structural relaxation times − spanning six decades as a function of temperature, density, and degree of confinement − collapse when plotted versus excess entropy. The data also collapse when plotted versus excess isochoric heat capacity, a behaviour that follows from the existence of isomorphs in the bulk and confined states.That confined liquids microscopically relax and flow with different characteristic time scales than bulk liquids is hardly surprising. Confining boundaries bias the spatial distribution of the constituent molecules and the ways by which those molecules can dynamically rearrange. These effects play important roles in the design of coating, nanopatterning, and nanomanufacturing technologies 1,2 . As a result, they have already been experimentally characterized for a wide variety of material systems, including small-molecule fluids 3-10 , polymers 11-16 , ionic liquids 17 , liquid crystals 18 , and dense colloidal suspensions [19][20][21][22][23] , and studied extensively via molecular simulations 22,24-31 . Recent reviews of confined-liquid behavior may be found in, e.g., Refs. 10,32 .Unfortunately, successful theories for predicting the dynamics of inhomogeneous fluids have been slower to emerge. Here, we explore the possibility of a novel approach for predicting how confinement affects the dynamics of viscous fluids. The central idea is motivated by the observation from molecular simulations that, under equilibrium conditions, key dimensionless "reduced" quantities for confined fluids closely correspond to those of homogeneous bulk fluids with the same excess entropy 33-37 (relative to an ideal gas at the same density and temperature). The excess entropy can be computed using Monte Carlo methods 36 or predicted from classical density-functional theories 35,38 . An open question is whether this observed correspondence between dynamics and excess entropy applies for fluids in deeply supercooled liquid states approaching the glass transition, where highly nontrivial dynamic effects of confinement are observed. Another open question is whether thermodynamic properties other than the excess entropy can be used to predict the dynamics in confinement.To investigate these questions we study the behavior of a glass-former comprising asymmetric dumbbell-shaped molecules 39 . This model is perhaps the simplest singlecomponent system that avoids freezing upon cooling or compression in confinement, allowing for a systematic comparison of the properties of supercooled states in both bulk and confined geometries. The latter is modeled as a slit-pore, i.e., a sandwich geometry, using a 9-3 LennardJo...

RUMD is a general purpose, high-performance molecular dynamics (MD) simulation package running on graphical processing units (GPU's). RUMD addresses the challenge of utilizing the many-core nature of modern GPU hardware when simulating small to medium system sizes (roughly from a few thousand up to hundred thousand particles). It has a performance that is comparable to other GPU-MD codes at large system sizes and substantially better at smaller sizes. RUMD is open-source and consists of a library written in C++ and the CUDA extension to C, an easy-to-use Python interface, and a set of tools for set-up and post-simulation data analysis. The paper describes RUMD's main features, optimizations and performance benchmarks.

The noncrystalline glassy state of matter plays a role in virtually all fields of materials science and offers complementary properties to those of the crystalline counterpart. The caveat of the glassy state is that it is out of equilibrium and therefore exhibits physical aging, i.e., material properties change over time. For half a century, the physical aging of glasses has been known to be described well by the material-time concept, although the existence of a material time has never been directly validated. We do this here by successfully predicting the aging of the molecular glass 4-vinyl-1,3-dioxolan-2-one from its linear relaxation behavior. This establishes the defining property of the material time. Via the fluctuation-dissipation theorem, our results imply that physical aging can be predicted from thermal-equilibrium fluctuation data, which is confirmed by computer simulations of a binary liquid mixture.