Walter Kob

Bio: Walter Kob is an academic researcher from University of Montpellier. The author has contributed to research in topics: Relaxation (physics) & Glass transition. The author has an hindex of 64, co-authored 293 publications receiving 15308 citations. Previous affiliations of Walter Kob include University of Mainz & Institut Universitaire de France.

Testing mode-coupling theory for a supercooled binary Lennard-Jones mixture I: The van Hove correlation function.

Abstract: We report the results of a large scale computer simulation of a binary supercooled Lennard-Jones liquid. We find that at low temperatures the curves for the mean squared displacement of a tagged particle for different temperatures fall onto a master curve when they are plotted versus rescaled time tD(T), where D(T) is the diffusion constant. The time range for which these curves follow the master curve is identified with the \ensuremath{\alpha}-relaxation regime of mode-coupling theory (MCT). This master curve is fitted well by a functional form suggested by MCT. In accordance with idealized MCT, D(T) shows a power-law behavior at low temperatures. The critical temperature of this power law is the same for both types of particles, and also the critical exponents are very similar. However, contrary to a prediction of MCT, these exponents are not equal to the ones determined previously for the divergence of the relaxation times of the intermediate scattering function [Phys. Rev. Lett. 73, 1376 (1994)]. At low temperatures, the van Hove correlation function (self as well as distinct part) shows almost no sign of relaxation in a time interval that extends over about three decades in time. This time interval can be interpreted as the \ensuremath{\beta}-relaxation regime of MCT. From the investigation of these correlation functions, we conclude the hopping processes are not important on the time scale of the \ensuremath{\beta} relaxation for this system and for the temperature range investigated. We test whether the factorization property predicted by MCT holds and find that this is indeed the case for all correlation functions investigated. The distance dependences of the critical amplitudes are in qualitative agreement with the ones predicted by MCT for some other mixtures. The non-Gaussian parameter for the self part of the van Hove correlation function for different temperatures follows a master curve when plotted against time t.

Dynamical heterogeneities in a supercooled lennard-jones liquid

Abstract: We present the results of a molecular dynamics computer simulation study in which we investigate whether a supercooled Lennard-Jones liquid exhibits dynamical heterogeneities. We evaluate the non-Gaussian parameter for the self part of the van Hove correlation function and use it to identify {open_quotes}mobile{close_quotes} particles. We find that these particles form clusters whose sizes grow with decreasing temperature. We also find that the relaxation time of the mobile particles is significantly shorter than that of the average particle, and that this difference increases with decreasing temperature. {copyright} {ital 1997} {ital The American Physical Society}

Stringlike cooperative motion in a supercooled liquid

Abstract: Extensive molecular dynamics simulations are performed on a glass-forming Lennard-Jones mixture to determine the nature of the cooperative motions occurring in this model fragile liquid. We observe stringlike cooperative molecular motion (``strings'') at temperatures well above the glass transition. The mean length of the strings increases upon cooling, and the string length distribution is found to be nearly exponential.

Cooling-rate effects in amorphous silica: A computer-simulation study.

Abstract: Using molecular-dynamics computer simulations we investigate how in silica the glass transition and the properties of the resulting glass depend on the cooling rate with which the sample is cooled. By coupling the system to a heat bath with temperature ${\mathit{T}}_{\mathit{b}}$(t), we cool the system linearly in time, ${\mathit{T}}_{\mathit{b}}$(t)=${\mathit{T}}_{\mathit{i}}$-\ensuremath{\gamma}t, where \ensuremath{\gamma} is the cooling rate. In qualitative accordance with experiments, the temperature dependence of the density shows a local maximum, which becomes more pronounced with decreasing cooling rate. We find that the glass transition temperature ${\mathit{T}}_{\mathit{g}}$ is in accordance with a logarithmic dependence on \ensuremath{\gamma}. The enthalpy, density, and thermal expansion coefficient for the glass at zero temperature decrease with decreasing \ensuremath{\gamma}. We show that also microscopic quantities, such as the radial distribution function, the bond-bond angle distribution function, the coordination numbers, and the distribution function for the size of the rings, depend significantly on \ensuremath{\gamma}. We demonstrate that the cooling-rate dependence of these microscopic quantities is significantly more pronounced than the one of macroscopic properties. Furthermore, we show that these microscopic quantities, as determined from our simulation, are in good agreement with the ones measured in real experiments, thus demonstrating that the used potential is a good model for silica glass. The vibrational spectrum of the system also shows a significant dependence on the cooling rate and is in qualitative accordance with the one found in experiments. Finally we investigate the properties of the system at finite temperatures in order to understand the microscopic mechanism for the density anomaly. We show that the anomaly is related to a densification and subsequent opening of the tetrahedral network when the temperature is decreased, whereas the distance between nearest neighbors, i.e., the size of the tetrahedra, does not change significantly. \textcopyright{} 1996 The American Physical Society.

Scaling Behavior in the β -Relaxation Regime of a Supercooled Lennard-Jones Mixture

Abstract: We have performed molecular dynamics simulations of a supercooled atomic liquid. The self-intermediate-scattering function in the $\ensuremath{\beta}$-relaxation regime has a power-law time dependence and temperature dependence consistent with the mode-coupling-theory prediction of a von Schweidler law, with exponents that are very close to satisfying the exponent relationship predicted by the theory. The diffusion constants have a power-law dependence on temperature with the same critical temperature. The exponents for diffusion differ from those of the relaxtion time, a result that is in disagreement with the theory.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Phd by thesis

Abstract: Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These