Comparative atomistic and coarse-grained study of water: What do we lose by coarse-graining?

We employ the inverse Boltzmann method to coarse-grain three commonly used three-site water models (TIP3P, SPC and SPC/E) where one molecule is replaced with one coarse-grained particle with isotropic two-body interactions only. The shape of the coarse-grained potentials is dominated by the ratio of two lengths, which can be rationalized by the geometric constraints of the water clusters. It is shown that for simple two-body potentials either the radial distribution function or the geometrical packing can be optimized. In a similar way, as needed for multiscale methods, either the pressure or the compressibility can be fitted to the all atom liquid. In total, a speed-up by a factor of about 50 in computational time can be reached by this coarse-graining procedure.     

Physisorption kinetics of electrons at plasma boundaries

Plasma-boundaries floating in an ionized gas are usually negatively charged. They accumulate electrons more efficiently than ions leading to the formation of a quasi-stationary electron film at the boundaries. We propose to interpret the build-up of surface charges at inert plasma boundaries, where other surface modifications, for instance, implantation of particles and reconstruction or destruction of the surface due to impact of high energy particles can be neglected, as a physisorption process in front of the wall. The electron sticking coefficient s_e and the electron desorption time τ_e, which play an important role in determining the quasi-stationary surface charge, and about which little is empirically and theoretically known, can then be calculated from microscopic models for the electron-wall interaction. Irrespective of the sophistication of the models, the static part of the electron-wall interaction determines the binding energy of the electron, whereas inelastic processes at the wall determine s_e and τ_e. As an illustration, we calculate s_e and τ_e for a metal, using the simplest model in which the static part of the electron-metal interaction is approximated by the classical image potential. Assuming electrons from the plasma to loose (gain) energy at the surface by creating (annihilating) electron-hole pairs in the metal, which is treated as a jellium half-space with an infinitely high workfunction, we obtain s_e≈10^-4 and τ_e≈10^-2 s. The product s_eτ_e≈10^-6 s has the order of magnitude expected from our earlier results for the charge of dust particles in a plasma but individually s_e is unexpectedly small and τ_e is somewhat large. The former is a consequence of the small matrix elements occurring in the simple model while the latter is due to the large binding energy of the electron. More sophisticated theoretical investigations, but also experimental support, are clearly needed because if s_e is indeed as small as our exploratory calculation suggests, it would have severe consequences for the understanding of the formation of surface charges at plasma boundaries. To identify what we believe are key issues of the electronic microphysics at inert plasma boundaries and to inspire other groups to join us on our journey is the purpose of this colloquial presentation.     

First-principles study of structrural and corrected band properties of wurtzite Zn1-xCdxO and Zn1-xMgxO systems

A first-principles method based on density functional theory(DFT),a generalized gradient approximation(GGA),and a projector-augmented wave(PAW) are used to study the structual and band properties of wurtzite Zn1-xCdxO and Zn1-xMgxO(0 ≤x≤1) ternary alloys.By taking into account all of the possible structures,the band gaps of Zn1-xCdxO and Zn1-xMgxO alloys are corrected and compared with experimental data.     

Electronic properties close to Dirac cone in two-dimensional organic conductor α-(BEDT-TTF)<Subscript>2</Subscript>I<Subscript>3</Subscript>

A zero-gap state (ZGS) has been found in a bulk system of two-dimensional organic conductor, α-(BEDT-TTF)₂I₃ salt which consists of four sites of donor molecules in a unit cell. In the present paper, the characteristic of the ZGS is analyzed in detail and the electronic properties are examined in the vicinity of the Dirac point where the conduction and valence bands degenerate to form the zero-gap. The eigenvectors of the energy band have four components of respective sites, where two of them correspond to inequivalent sites and the other two correspond to equivalent sites. It is shown that the former exhibits an exotic momentum dependence around the contact point and the latter shows almost a constant dependence. The density of states of each site close to the Dirac point is calculated to demonstrate the temperature dependence of the local magnetic susceptibility and the local nuclear magnetic relaxation rate. Further, the robust property of the ZGS against the anion potential is also shown by using the second-order perturbation.     

Stochastic resonance in an ion channel following the non-Arrhenius gating rate

Stochastic resonance (SR) is a novel cooperative phenomenon occurring in nonlinear systems due to coupling of an ambient noise and an external signal. Biological systems may use SR mechanism to detect the signal efficiently from an external environment. A number of studies have addressed the SR in artificial ion channels considering external voltages as noises. More important than these external noises is the internal, thermal noise which changes the channel conformations essential for biological functions. In this work, we consider that the channel gating rates follow a non-Arrhenius temperature dependence derived from experimental data of a real biological channel. Using the Monte-Carlo simulations, we find that in this channel SR occurs near a physiological temperature in a very distinctive manner compared with that for the Arrhenius gating model.     

Kolmogorov-Smirnov method for the determination of signal time-shifts

The intermediate-state interaction and the structure of amplitudes of complicated processes in the medium (decays, reactions and the n transitions) are studied. It is proposed to use the branching ratio of channels of the free-space hadron-nucleon interaction as a test in the construction and verification of the models. The corresponding formulas for the processes in the medium are obtained. The connection between the particle self-energy and amplitudes of subprocesses is analyzed as well.     

A study of the evaporative deposition process: Pipes and truncated transport dynamics

We consider contact line deposition of an evaporating thin drop. Following Dupont’s proposal (unpublished), we focus on transport dynamics truncated by a maximal concentration as the single deposition mechanism. The truncated transport process, formalized as the “pipe model”, admits a characteristic shock front that has a robust functional form and depends only on local hydrodynamic properties. By applying the pipe model, we solve the density profile in different asymptotic regimes. In particular, we find that near the contact line the density profile follows a scaling law that is proportional to the square root of the concentration ratio defined as the initial solute volume concentration divided by the maximal concentration. The maximal deposit density occurs at about 2/3 of the total drying time for uniform evaporation and 1/2 for diffusion-controlled evaporation. Away from the contact line, areal density decays exponentially with the radial distance to the power of -3 for the uniform evaporation and -7 for the diffusion-controlled evaporation.     

Elasticity transition and loop formation in vibrated bead chains: A simulation of polymer chains

By measuring the distribution function of the end-to-end distance, we find that strongly shaken bead chains exhibit many properties, such as the rigid-rod-to-Gaussian chain transition, scaling, fast drop of loop formation probability in the short-chain regime, and enhancement of loop formation probability for kinked chains, of long-chain polymers. Though there is difference in local details between our chains and the worm-like chains, our results are consistent with recent calculations based on the worm-like chain model in many respects.     

Hawking Radiation from Plane Symmetric Black Hole Covariant Anomaly

Based on the covariant anomaly cancellation method, which is believed to be more refined than the initial approach of Robinson and Wilczek, we discuss Hawking radiation from the plane symmetric black hole. The result shows that Hawking radiation from the non-spherical symmetric black holes also can be derived from the viewpoint of anomaly.