Theory and simulation of open systems out of equilibrium.

We consider the theoretical model of Bergmann and Lebowitz for open systems out of equilibrium and translate its principles in the adaptive resolution simulation molecular dynamics technique. We simulate Lennard-Jones fluids with open boundaries in a thermal gradient and find excellent agreement of the stationary responses with the results obtained from the simulation of a larger locally forced closed system. The encouraging results pave the way for a computational treatment of open systems far from equilibrium framed in a well-established theoretical model that avoids possible numerical artifacts and physical misinterpretations.

Nanoscale domains in ionic liquids: A statistical mechanics definition for molecular dynamics studies.

One of the many open questions concerning Ionic Liquids (ILs) is the existence of nanoscale supra-molecular domains which characterize the bulk. The hypothesis of their existence does not meet a general consensus since their definition seems to be based on ad hoc arbitrary criteria rather than on general and solid first principles of physics. In this work, we propose a suitable definition of supra-molecular domains based on first principles of statistical mechanics. Such principles can be realized through the application of a recently developed computational tool which employs adaptive molecular resolution. The method can identify the smallest region of a liquid for which the atomistic details are strictly required, while the exterior plays the role of a generic structureless thermodynamic reservoir. We consider four different imidazolium-based ILs and show that indeed one can quantitatively represent the liquid as a collection of atomistically self-contained nanodroplets embedded in a generic thermodynamic bath. Such nanodroplets express a characteristic length scale for heterogeneity in ILs.

Probing spatial locality in ionic liquids with the grand canonical adaptive resolution molecular dynamics technique.

We employ the Grand Canonical Adaptive Resolution Simulation (GC-AdResS) molecular dynamics technique to test the spatial locality of the 1-ethyl 3-methyl imidazolium chloride liquid. In GC-AdResS, atomistic details are kept only in an open sub-region of the system while the environment is treated at coarse-grained level; thus, if spatial quantities calculated in such a sub-region agree with the equivalent quantities calculated in a full atomistic simulation, then the atomistic degrees of freedom outside the sub-region play a negligible role. The size of the sub-region fixes the degree of spatial locality of a certain quantity. We show that even for sub-regions whose radius corresponds to the size of a few molecules, spatial properties are reasonably reproduced thus suggesting a higher degree of spatial locality, a hypothesis put forward also by other researchers and that seems to play an important role for the characterization of fundamental properties of a large class of ionic liquids.

Simulation of macromolecular liquids with the adaptive resolution molecular dynamics technique.

We extend the application of the adaptive resolution technique (AdResS) to liquid systems composed of alkane chains of different lengths. The aim of the study is to develop and test the modifications of AdResS required in order to handle the change of representation of large molecules. The robustness of the approach is shown by calculating several relevant structural properties and comparing them with the results of full atomistic simulations. The extended scheme represents a robust prototype for the simulation of macromolecular systems of interest in several fields, from material science to biophysics.

Adaptive resolution molecular dynamics simulation through coupling to an internal particle reservoir.

For simulation studies of (macro) molecular liquids it would be of significant interest to be able to adjust or increase the level of resolution within one region of space, while allowing for the free exchange of molecules between open regions of different resolution or representation. We generalize the adaptive resolution idea and suggest an interpretation in terms of an effective generalized grand canonical approach. The method is applied to liquid water at ambient conditions.

Quantum locality and equilibrium properties in low-temperature parahydrogen: a multiscale simulation study.

Parahydrogen is the spin-zero singlet state of molecular hydrogen, which at low temperature (between 14 and 25 K) is in a fluid state. A classical treatment of the system leads to unphysical freezing, and the inclusion of quantum delocalization of the molecule is then required to obtain a realistic description of its equilibrium properties. In the present work, we employ the classical-quantum adaptive resolution method AdResS to investigate the spatial extension of quantum delocalization effects in the bulk fluid at low temperature. Specifically, we simulate a small, spherical region of the system in full quantum detail: this region is coupled to a bulk of coarse-grained particles with classical, quantum-derived effective interactions obtained from quantum simulations. The two regions are interfaced through open boundaries and in conditions of thermodynamic equilibrium. Structural properties of the fluid, namely, pair distribution functions, are measured for different sizes of the quantum region. The results of this work show that, for the thermodynamic conditions corresponding to the range of temperature between 14 and 25 K, the bead-based, quantum structural properties of low-temperature parahydrogen are deemed local and do not require the support of an explicit quantum bulk.

Adaptive resolution simulation of liquid para-hydrogen: testing the robustness of the quantum-classical adaptive coupling.

Adaptive resolution simulations for classical systems are currently made within a reasonably consistent theoretical framework. Recently we have extended this approach to the quantum-classical coupling by mapping the quantum nature of an atom onto a classical polymer ring representation within the path integral approach [Poma & Delle Site, Phys. Rev. Lett., 2010, 104, 250201]. In this way the process of interfacing adaptively a quantum representation to a classical one corresponds to the problem of interfacing two regions with a different number of effective "classical" degrees of freedom; thus the classical formulation of the adaptive algorithm applies straightforwardly to the quantum-classical problem. In this work we show the robustness of such an approach for a liquid of para-hydrogen at low temperature. This system represents a highly challenging conceptual and technical test for the adaptive approach due to the extreme thermodynamical conditions where quantum effects play a central role.

Proton Wires via One-Dimensional Water Chains Adsorbed on Metallic Steps.

The process of proton transfer is here analyzed for one-dimensional water chains adsorbed on metallic steps. When the water chain contains a hydronium and a hydroxyl ion, two different mechanisms are possible, depending on the metal substrate. On coinage metals (Ag, Au), recombination is observed through a spontaneous Grotthuss mechanism. On more reactive surfaces (Pd and Pt), the hydronium ion is unstable and releases a proton that adsorbs onto the metal, leaving the negatively charged OH(-) unbalanced. In this case, the negative charge can be transferred along the wire with very low activation barriers.

Classical to path-integral adaptive resolution in molecular simulation: towards a smooth quantum-classical coupling.

Simulations that couple different molecular models in an adaptive way by changing resolution on the fly allow us to identify the relevant degrees of freedom of a system. This, in turn, leads to a detailed understanding of the essential physics which characterizes a system. While the delicate process of transition from one model to another is well understood for the adaptivity between classical molecular models the same cannot be said for the quantum-classical adaptivity. The main reason for this is the difficulty in describing a continuous transition between two different kinds of physical principles: probabilistic for the quantum and deterministic for the classical. Here we report the basic principles of an algorithm that allows for a continuous and smooth transition by employing the path integral description of atoms.

Electrostatic properties of liquid 1,3-dimethylimidazolium chloride: role of local polarization and effect of the bulk.

The molecular polarization in a 1,3-dimethylimidazolium chloride ([DMIM][Cl]) ionic liquid is analyzed for a bulk liquid via the Car-Parrinello approach. The analysis reveals that the electric dipole moments of cations and anions are characterized by large fluctuations, however these are primarily due to the molecules in the immediate surroundings. These results on one hand shed light on some basic physical and chemical features of this liquid, and on the other represent a fundamental handle for the development of accurate classical force fields; this aspect is extensively discussed and some suggestions are made.

Interacting electrons, spin statistics, and information theory.

We consider a nearly (or quasi) uniform gas of interacting electrons for which spin statistics play a crucial role. A previously developed procedure, based on the extension of the Levy-Lieb constrained search principle and Monte Carlo sampling of electron configurations in space, allows us to approximate the form of the kinetic-energy functional. For a spinless electron gas, this procedure led to a correlation term, which had the form of the Shannon entropy, but the resulting kinetic-energy functional does not satisfy the Lieb-Thirring inequality, which is rigorous and one of the most general relations regarding the kinetic energy. In this paper, we show that when the fermionic character of the electrons is included via a statistical spin approach, our procedure leads to correlation terms, which also have the form of the Shannon entropy and the resulting kinetic-energy functional does satisfy the Lieb-Thirring inequality. In this way we further strengthen the connection between Shannon entropy and electron correlation and, more generally, between information theory and quantum mechanics.

Polymers near metal surfaces: selective adsorption and global conformations.

We study the properties of a polycarbonate melt near a nickel surface as a model system for the interaction of polymers with metal surfaces by employing a multiscale modeling approach. For bulk properties, a suitably coarse-grained bead-spring model is simulated by molecular dynamics methods with model parameters directly derived from quantum chemical calculations. The surface interactions are parametrized and incorporated by extensive quantum mechanical density functional calculations using the Car-Parrinello method. We find strong chemisorption of chain ends, resulting in significant modifications of the melt composition when compared to an inert wall.