L. Delle Site

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.

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.

Structures of adsorbed water layers on MgO: an ab initio study

Abstract A systematic search using an ab initio density-functional method has been carried out for energy minima for a monolayer of water on MgO. Minima were sought in which one third of the water molecules were dissociated, and the observed p(3×2) symmetry satisfied. Six such minima were found, three of which are within 300 k B per water molecule of the lowest energy structure. We also found a structure with a similar energy with (2×2) symmetry and half the water molecules dissociated. The structures are stabilised by the donation of three hydrogen bonds to each hydroxide ion.

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.

Formulation of Liouville's theorem for grand ensemble molecular simulations.

Liouvilles theorem in a grand ensemble, that is for situations where a system is in equilibrium with a reservoir of energy and particles, is a subject that, to our knowledge, has not been explicitly treated in literature related to molecular simulation. Instead, Liouvilles theorem, a central concept for the correct employment of molecular simulation techniques, is implicitly considered only within the framework of systems where the total number of particles is fixed. However, the pressing demand of applied science in treating open systems leads to the question of the existence and possible exact formulation of Liouvilles theorem when the number of particles changes during the dynamical evolution of the system. The intention of this paper is to stimulate a debate about this crucial issue for molecular simulation.Liouvilles theorem in a grand ensemble, that is for situations where a system is in equilibrium with a reservoir of energy and particles, is a subject that, to our knowledge, has not been explicitly treated in literature related to molecular simulation. Instead, Liouvilles theorem, a central concept for the correct employment of molecular simulation techniques, is implicitly considered only within the framework of systems where the total number of particles is fixed. However, the pressing demand of applied science in treating open systems leads to the question of the existence and possible exact formulation of Liouvilles theorem when the number of particles changes during the dynamical evolution of the system. The intention of this paper is to stimulate a debate about this crucial issue for molecular simulation.

Dynamic surface decoupling in a sheared polymer melt

We propose that several mechanisms contribute to friction in a polymer melt adsorbed at a structured surface. The first one is the well-known disentanglement of bulk polymer chains from the surface layer. However, if the surface is ideal at the atomic scale, the adsorbed parts of polymer chains can move along the equipotential lines of the surface potential. This gives rise to a strong slippage of the melt. For high shear rates chains partially desorb. However, the friction force on adsorbed chains increases, resulting in quasi-stick boundary conditions. We propose that the adsorbed layers can be efficiently used to adjust the friction force between the polymer melt and the surface.

Specific interaction of polymers with surface defects: structure formation of polycarbonate on nickel

By combining quantum ab initio calculations and coarse grained polymer models we employ a multiscale approach to study site specific adsorption of polymers (polycarbonate) to a step on an otherwise perfectly flat nickel (111) surface. The presence of the defect leads to a well defined chain localization and ordering at the surface. This can be taken as a model situation for both surface site as well as chemical group (within the chain monomer) specific interaction of macromolecules with metal surfaces. The results shed some light on important possible mechanisms occurring at step defects and corners used to control mineralization processes and thus material properties.

What can classical simulators learn from ab initio simulations

We compare the electrostatic properties of individual water molecules in ab initio simulations and in some potential models used in classical simulations. We find that the standard rigid water models at room temperature and ambient pressure describe the average electrostatic properties of molecules quite well, although there are considerable fluctuations. Preliminary results show that the dipole moment of water molecules near either a sodium ion or a methane molecule are not significantly different from its average value in bulk water, which may justify the use of non-polarisable water models in simulations of biological molecules. In ice, however, the dipole induced by the surroundings is appreciably higher. Incorrect freezing points can be attributed to a bad description of the solid phase rather than to a bad description of the liquid phase.

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.

A procedure for calculating the many-particle Bohm quantum potential.

In a recent work, M.Kohout (Int. J. Quant. Chem. 87 (2002) 12) raised the important question of how to make a correct use of Bohms approach for defining a quantum potential. In this work, by taking into account Kohouts results, we propose a general self-consistent iterative procedure for solving this problem.