Quasi-bound atoms in collective dynamics of liquid Sb.

We report anab initiosimulation and theoretical study of collective dynamics in liquid Sb at 973 K. An application of the GCM (generalized collective modes) theoretical approach to analysis of simulation-derived time correlation functions resulted in two types of propagating eigenmodes. We found that the almost flat dispersion of the high-frequency branch of propagating modes can be explained by out-of-phase oscillations of nearest neighbors which form quasi-bound atomic pairs for at least 30 ps. We discuss the features of collective dynamics in non-simple metallic melts containing quasi-bound pairs.

Comment on "Lagrangian formulation and symmetrical description of liquid dynamics".

Trachenko [Trachenko, Phys. Rev. E 96, 062134 (2017)2470-004510.1103/PhysRevE.96.062134] has argued for creating a Lagrangian formulation of liquid dynamics. This Comment shows that the proposed two-field Lagrangian does not describe dynamics of liquid in a correct way. Other claims on the symmetric description of liquid dynamics and on the similarity of Maxwell and Frenkel characteristic times are discussed.

A simple closure procedure for the study of velocity autocorrelation functions in fluids as a "bridge" between different theoretical approaches.

Velocity autocorrelation functions (VAFs) of the fluids are studied on short- and long-time scales within a unified approach. This approach is based on an effective summation of the infinite continued fraction at a reasonable assumption about convergence of relaxation times of the high order memory functions, which have a purely kinetic origin. The VAFs obtained within our method are compared with computer simulation data for the liquid Ne at different densities and the results, which follow from the Markovian approximation for the highest order kinetic kernels. It is shown that in all the thermodynamic points and at the chosen level of the hierarchy, our results agree much better with the molecular dynamic data than those of the Markovian approximation. The density dependence of the transition time, needed for the fluid to attain the hydrodynamic stage of evolution, is evaluated. The common and distinctive features of our method are discussed in their relations to the generalized collective mode theory, the mode coupling theory, and some other theoretical approaches.

Behavior of Supercritical Fluids across the "Frenkel Line".

The "Frenkel line" (FL), the thermodynamic locus where the time for a particle to move by its size equals the shortest transverse oscillation period, has been proposed as a boundary between recently discovered liquid-like and gas-like regions in supercritical fluids. We report a simulation study of isothermal supercritical neon in a range of densities intersecting the FL. Specifically, structural properties and single-particle and collective dynamics are scrutinized to unveil the onset of any anomalous behavior at the FL. We find that (i) the pair distribution function smoothly evolves across the FL displaying medium-range order, (ii) low-frequency transverse excitations are observed below the "Frenkel frequency", and (iii) the high-frequency shear modulus does not vanish even for low-density fluids, indicating that positive sound dispersion characterizing the liquid-like region of the supercritical state is unrelated to transverse dynamics. These facts critically undermine the definition of the FL and its significance for any relevant partition of the supercritical phase.

Dynamics and Thermodynamics beyond the critical point.

Sudden changes in the dynamical properties of a supercritical fluid model have been found as a function of pressure and temperature (T/T(c) = 2-5 and P/P(c) = 10-10(3)), striving with the notion of a single phase beyond the critical point established by thermodynamics. The sound propagation in the Terahertz frequency region reveals a sharp dynamic crossover between the gas like and the liquid like regimes along several isotherms, which involves, at sufficiently low densities, the interplay between purely acoustic waves and heat waves. Such a crossover allows one to determine a dynamic line in the phase diagram which exhibits a very tight correlation with a number of thermodynamic observables, showing that the supercritical state is remarkably more complex than thought so far.

Shear relaxation in iron under the conditions of earth's inner core.

Large scale molecular dynamics simulations of iron at high pressure and temperature are performed to investigate the physics of shear softening. A solid 16×10(6) atoms sample of iron is grown out of the liquid with a small solid immersed in it at the start of simulation. We observe that diffusion in the sheared solid is similar to that in liquid, even though at different time scales. This allows us to describe the time dependence of shear stress in terms of elastic and hydrodynamic relaxation. The elastic response of the sample is close to the elastic response of Earth's inner core. This explains the abnormally low shear modulus in the core. The reason for the low shear modulus is the presence of defects of the crystal structure.

Primitive model for cation hydrolysis: a molecular-dynamics study.

A model of primitive cation MZ+ in water is introduced in order to clarify the influence of ion charge on the hydration structure and dynamic properties of highly charged cations in aqueous solutions. A flexible nonconstrained model for water molecules is used. The considered model in the case of monovalent cation M+ reduces to the realistic model for the hydration structure of Na+. It is shown that for divalent ion M2+ the strong cation-water electrostatic interaction leads to the formation of stable structures constituted by six water molecules octahedrally arranged around the cation. The cation-oxygen attraction and cation-hydrogen repulsion modify the octahedral hydration configuration of the model cations M3+ and M4+ and additional water molecules can join the hydration shell. The increase of cation charge results in the increase of O-H bond length of water molecules in the cation hydration shell. Further increase of ion-water electrostatic interaction causes the loss of some protons from the hydration shell of cations M4+, M5+, and M6+ that is interpreted as a cation hydrolysis effect. For a correct description of this phenomenon the considered model is improved by modeling the effects of the charge redistribution between hydrolysis products, which essentially modified and stabilized the hydrated-hydrolyzed structure of cation. The influence of cation charge on dynamical properties of cation MZ+ and oxygens in its hydration shell was investigated and analyzed.

Free energy of solvation of simple ions: molecular-dynamics study of solvation of Cl- and Na+ in the ice/water interface.

Molecular-dynamics simulations of Cl(-) and Na(+) ions are performed to calculate ionic solvation free energies in both bulk simple point-charge/extended water and ice 1 h at several different temperatures, and at the basal ice 1 h/water interface. For the interface we calculate the free energy of "transfer" of the ions across the ice/water interface. For the ions in bulk water in the NPT ensemble at 298 K and 1 atm, results are found to be in good agreement with experiments, and with other simulation results. Simulations performed in the NVT ensemble are shown to give equivalent solvation free energies, and this ensemble is used for the interfacial simulations. Solvation free energies of Cl(-) and Na(+) ions in ice at 150 K are found to be approximately 30 and approximately 20 kcal mol(-1), respectively, less favorable than for water at room temperature. Near the melting point of the model the solvation of the ions in water is the same (within statistical error) as that measured at room temperature, and in the ice is equivalent and approximately 10 kcal mol(-1) less favorable than the liquid. The free energy of transfer for each ion across ice/water interface is calculated and is in good agreement with the bulk observations for the Cl(-) ion. However, for the model of Na(+) the long-range electrostatic contribution to the free energy was more negative in the ice than the liquid, in contrast with the results observed in the bulk calculations.

Methemoglobinemia induced by refrigerated vegetable puree in conjunction with supraventricular tachycardia.

UNLABELLED: We report on a case of methemoglobinemia in conjunction with supraventricular tachycardia. A 6-mo-old infant was admitted to hospital with perioral cyanosis three hours after eating a refrigerated mixed-vegetable puree. The patient developed supraventricular tachycardia (SVT) and had a heart rate of 230 beats/min. The arrhythmia resolved spontaneously. A chemical analysis showed oxygen saturation of 85% and a methemoglobin level of 25%. The infant was treated with oxygen and referred to the Pediatric Intensive Care Unit. The patient had two additional short episodes of SVT, which were resolved spontaneously without treatment. She received oxygen for 8 h until the methemoglobin level dropped to 1%. CONCLUSION: Long-term storage of refrigerated vegetables can cause methemoglobinemia in infants. Methemoglobinemia can appear in conjunction with SVT, but it is also possible that in a child with a tendency to develop SVT, methemoglobinemia involved hypoxemia, which then triggers SVT.

Collective dynamics in liquid lead. II. Mode contributions to time correlation functions.

Mode contributions of different collective excitations to the density-density time correlation function in liquid lead are studied within the nine-variable approach of generalized collective modes. It is shown, that a kinetic relaxing mode, caused by slow density fluctuations, defines almost completely the shape of density-density time correlation function for wave numbers close to the main peak position of the static structure factor. The physical meaning of this mode is discussed.

Collective dynamics in liquid lead: generalized propagating excitations.

A microscopic approach to the investigation of generalized collective excitations, developed recently for pure liquids, is applied to the study of the spectrum of collective excitations in a liquid metal. The calculations are performed for liquid lead at two temperatures (above the melting point and in high-temperature region) and the results are compared. From the analysis of spectra, obtained for different basis sets of dynamical variables, we conclude that there exist three branches of propagating collective excitations, which correspond to sound and heat (high- and low-frequency) waves in the liquid. It is shown that the branch of low-frequency heat waves contains a propagation gap in the hydrodynamic region. An analytical expression for the width of the propagation gap for low-frequency heat waves is derived.

Generalized hydrodynamics of binary liquids: transverse collective modes

The parameter-free generalized collective modes approach in eight-variable approximation is applied for investigation of transverse dynamics of Lennard-Jones liquid Kr-Ar beyond the hydrodynamic region. We find four branches of propagating eigenmodes in the spectrum of transverse collective excitations. Different basis sets of dynamical variables are applied to estimate the origin of different branches in the spectrum. It is shown that for large wave numbers the general feature of transverse collective excitations is their "partial" character, while in hydrodynamic limit they are formed by collective behavior of liquid. A detailed analysis of the separated contributions from different collective modes into time correlation functions and spectral functions is performed. The condition of existence of high-frequency mass-concentration waves is derived. It is shown that high-frequency collective excitations, caused by the mass-concentration fluctuations, reflect some properties of optic phonon modes in solids.

Acute acalculous cholecystitis caused by Salmonella typhi in a 6-year-old child.

A rare case of acute acalculous cholecystitis caused by Salmonella typhi in a 6-year-old child is presented. The clinical signs were fulminant, with diffuse peritonitis being suspected. Cholecystostomy and i.v. ceftriaxone proved efficacious and the girl was discharged in less than two weeks. The appropriate literature is reviewed.