Saddle Points and Dynamics of Lennard-Jones Clusters, Solids and Supercooled Liquids

The properties of higher-index saddle points have been invoked in recent theories of the dynamics of supercooled liquids. Here we examine in detail a mapping of configurations to saddle points using minimization of $|\nabla E|^2$, which has been used in previous work to support these theories. The examples we consider are a two-dimensional model energy surface and binary Lennard-Jones liquids and solids. A shortcoming of the mapping is its failure to divide the potential energy surface into basins of attraction surrounding saddle points, because there are many minima of $|\nabla E|^2$ that do not correspond to stationary points of the potential energy. In fact, most liquid configurations are mapped to such points for the system we consider. We therefore develop an alternative route to investigate higher-index saddle points and obtain near complete distributions of saddles for small Lennard-Jones clusters. The distribution of the number of stationary points as a function of the index is found to be Gaussian, and the average energy increases linearly with saddle point index in agreement with previous results for bulk systems.Comment: 14 pages, 7 figure

Exhaust of Underexpanded Jets from Finite Reservoirs

We examine the response of an underexpanded jet to a depleting, finite reservoir with experiments and simulations. An open-ended shock tube facility with variable reservoir length is used to obtain images of nitrogen and helium jet structures at successive instances during the blowdown from initial pressure ratios of up to 250. The reservoir and ambient pressures are simultaneously measured to obtain the instantaneous pressure ratio. We estimate the time-scales for jet formation and reservoir depletion as a function of the specific heat ratio of the gas and the initial pressure ratio. The jet structure formation time-scale is found to become approximately independent of pressure ratio for ratios greater than 50. In the present work, no evidence of time-dependence in the Mach disk shock location is observed for rates of pressure decrease associated with isentropic blowdown of a finite reservoir while the pressure ratio is greater than 15. The shock location in the finite- reservoir jet can be calculated from an existing empirical fit to infinite-reservoir jet data evaluated at the instantaneous reservoir pressure. For pressure ratios below 15, however, the present data deviate from a compilation of data for infinite-reservoir jets. A new fit is obtained to data in the lower pressure regime. The self-similarity of the jet structure is quantified and departure from similarity is noted to begin at pressure ratios lower than about 15, approximately the same ratio which limits existing empirical fits

Monitoring of mercury content in agroecosystems of the central Chernozem region of Russia

The paper considers the results of the local agroecological monitoring of the agroecosystems of the Belgorod region and the ecosystems of Yamskaya Steppe of the reserve “Belogorye

СОВРЕМЕННОЕ СОСТОЯНИЕ ПРОИЗВОДСТВА ОВОЩЕЙ В РОССИЙСКОЙ ФЕДЕРАЦИИ

The current state of vegetable production in Russia is discussed in the article. Comparative assessment of the dynamics of vegetable production in the agricultural organizations, farms and private farms since 2008 has been conducted. The contribution of these organizations in the bulk yield of vegetable crops on an annual basis in each regions is presented. The private farms produce up to 80% of the bulk yield of vegetables. Such rearrangement of production has negatives and benefits. This production is low-gain and oriented toward to domestic consumption. In 2012 and 2013, the production of vegetable crops was almost equal and accounted more than 14 million tons.В статье проанализировано современное состояние овощеводства в Российской Федерации. Проведена сравнительная оценка динамики производства овощей в сельскохозяйственных организациях, крестьянских (фермерских) хозяйствах и хозяйствах населения с 2008 года, показан вклад этих организаций в структуре валового сбора овощей по годам и регионам. Основной объем производства овощей сосредоточен в личных подсобных хозяйствах, которые дают до 80% этой продукции. Такое перераспределение производства имеет как положительные, так и отрицательные моменты: оно стало низкорентабельным, основная масса продукции используется для внутреннего потребления, население потребляет овощи крайне неравномерно в течение года и в зависимости от региона.  По данным органов управления АПК субъектов, производство овощных культур в 2013 году во всех категориях хозяйств Российской Федерации составило 14,689 млн т, или 100,3% к уровню 2012 года (14,62 млн т)

Traveling through potential energy landscapes of disordered materials: the activation-relaxation technique

A detailed description of the activation-relaxation technique (ART) is presented. This method defines events in the configurational energy landscape of disordered materials, such as a-Si, glasses and polymers, in a two-step process: first, a configuration is activated from a local minimum to a nearby saddle-point; next, the configuration is relaxed to a new minimum; this allows for jumps over energy barriers much higher than what can be reached with standard techniques. Such events can serve as basic steps in equilibrium and kinetic Monte Carlo schemes.Comment: 7 pages, 2 postscript figure

Tendency to occupy a statistically dominant spatial state of the flow as a driving force for turbulent transition

A simple analytical model for a turbulent flow is proposed, which considers the flow as a collection of localized spatial structures that are composed of elementary "cells" in which the state of the particles (atoms or molecules) is uncertain. The Reynolds number is associated with the ratio between the total phase volume for the system and that for the elementary cell. Calculating the statistical weights of the collections of the localized structures, it is shown that as the Reynolds number increases, the elementary cells group into the localized structures, which successfully explains the onset of turbulence and some other characteristic properties of turbulent flows. It is also shown that the basic assumptions underlying the model are involved in the derivation of the Navier-Stokes equation, which suggests that the driving force for the turbulent transition described with the hydrodynamic equations is essentially the same as in the present model, i.e. the tendency of the system to occupy a statistically dominant state plays a key role. The instability of the flow can then be a mechanism to initiate the structural rearrangement of the flow to find this state.Comment: 7 pages, 4 figures, and Supplementary Material (2 pages, 3 figures), to be submitted to "Foundations of Physics". arXiv admin note: substantial text overlap with arXiv:1102.515

Effect of Secondary Echo Signals in Spin-Systems with a Large Inhomogeneous Broadening of NMR Line

The possibility of comparatively simple and fast determination of characteristic relaxation parameters T1, T2 and T3 for nuclear spin-systems with strong Larmor and Rabi inhomogeneous broadenings of NMR lines using the secondary echo signal effect was experimentally shown. Resides, this method gives opportunity to obtain a valuable infomation on the inhomogeneous NMR broadening which reflects the character of magnetic field microscopic destribution in such systems, as example, multidomain magnetics and superconductors.Comment: 12 pages, 5 figure

К вопросу о назначении дополнительных остановок пассажирским поездам дальнего следования в границах города Москвы

In many world’s most populous cities, railways are an integral part of urban transport systems. Commuter and intraurban passenger railway traffic is often and widely considered in this context. On the other hand, the constant growth in passenger traffic of longdistance trains, traditionally gravitating towards railway stations and adjacent urban transport hubs, including metro stations through which arriving passengers pass towards their final destinations in the city, requires search for new solutions to improve comfort of the trip for passengers, and to remove the excess load from the urban transit system. Considering the experience and features of organization of long-distance passenger railway traffic in various countries, the authors based on previous research suggests certain solutions regarding Moscow railway hub. To organize comfortable environment for passengers of all categories of trains, it is advisable to develop a balanced technology of train traffic within the Moscow junction, excluding overloading of individual elements of the transport system (in particular, of rail stations in the city center). The objective of the article is to analyze global experience in organizing traffic of long-distance passenger trains within the boundaries of large agglomerations and to develop proposals to organize additional stopping points for long-distance trains in various districts of Moscow, which will help reduce the load on central transport and interchange hubs, improve quality of transportation services for passengers, and develop the districts of Moscow and Moscow region adjacent to the new TIHs.Во многих крупных городах мира железные дороги являются неотъемлемым элементом городских транспортных систем. Наиболее часто и широко рассматриваются в этом контексте пригородное и внутригородское пассажирское железнодорожное движение. С другой стороны, постоянный рост пассажиропотоков поездов дальнего следования, традиционно тяготеющих к железнодорожным вокзалам и прилегающим к ним городским транспортным узлам, в том числе станциям метро, через которые прибывающие пассажиры отправляются к конечным точкам назначения в городе, требует поиска новых решений, позволяющих как повысить комфортность поездки для пассажиров, так и снять избыточную нагрузку с городской транспортной системы. С учётом опыта и специфики организации дальнего пассажирского железнодорожного сообщения в различных странах, в статье на основе ранее проведённых исследований, в первую очередь, предлагаются модели решений для московского железнодорожного узла, который является сложным транспортным комплексом. С целью организации комфортных условий для пассажиров всех категорий поездов необходимо разработать сбалансированную технологию движения поездов в узле, исключающую перегрузку отдельных элементов транспортной системы (в частности, вокзалов в центре города). Целью данной статьи является анализ зарубежного опыта организации движения пассажирских поездов дальнего следования в границах крупных агломераций и выработка предложений по организации дополнительных остановочных пунктов поездам дальнего следования в различных районах Москвы, которые помогут снизить нагрузку на центральные транспортно-пересадочные узлы, повысить качество транспортного обслуживания пассажиров и вместе с тем развить прилегающие к новым транспортно-пересадочным узлам районы Москвы и Московской области

Exploring the Free Energy Landscape: From Dynamics to Networks and Back

The knowledge of the Free Energy Landscape topology is the essential key to understand many biochemical processes. The determination of the conformers of a protein and their basins of attraction takes a central role for studying molecular isomerization reactions. In this work, we present a novel framework to unveil the features of a Free Energy Landscape answering questions such as how many meta-stable conformers are, how the hierarchical relationship among them is, or what the structure and kinetics of the transition paths are. Exploring the landscape by molecular dynamics simulations, the microscopic data of the trajectory are encoded into a Conformational Markov Network. The structure of this graph reveals the regions of the conformational space corresponding to the basins of attraction. In addition, handling the Conformational Markov Network, relevant kinetic magnitudes as dwell times or rate constants, and the hierarchical relationship among basins, complete the global picture of the landscape. We show the power of the analysis studying a toy model of a funnel-like potential and computing efficiently the conformers of a short peptide, the dialanine, paving the way to a systematic study of the Free Energy Landscape in large peptides.Comment: PLoS Computational Biology (in press

Atomic layering at the liquid silicon surface: a first- principles simulation

We simulate the liquid silicon surface with first-principles molecular dynamics in a slab geometry. We find that the atom-density profile presents a pronounced layering, similar to those observed in low-temperature liquid metals like Ga and Hg. The depth-dependent pair correlation function shows that the effect originates from directional bonding of Si atoms at the surface, and propagates into the bulk. The layering has no major effects in the electronic and dynamical properties of the system, that are very similar to those of bulk liquid Si. To our knowledge, this is the first study of a liquid surface by first-principles molecular dynamics.Comment: 4 pages, 4 figures, submitted to PR