G. G. Malenkov

Spatial localization and dynamics of water molecules with good tetrahedral surroundings

The local structure of the molecular- dynamic model of water (729 particles at 300 K) is analyzed by isolating molecules whose surroundings differ slightly in configuration from a regular tetrahedron. These molecules are not randomly distributed in space but form nanometer clusters having a fractal structure. In these clusters, molecules are less mobile than the model molecules in general;their self- correlation function of rate and the density of vibrational states also differ from the average characteristics of the system.

Collective motions in computer models of water. Large-scale and long-time correlations

Two-particle correlation functions describing the simultaneous motion of a pair of molecules initially separated by a given distance R0 are calculated to study collective effects in the diffusive motion of water molecules in molecular dynamics models. Various types of such functions and their dependences on the interaction potential, temperature, and the number of particles in the model are considered. At short times (of the order of ten picoseconds), these functions exhibit irregular behavior depending on R0. The most nontrivial and unexpected result was the detection of correlations in the displacements of pairs of particles that extend for tens of angstroms and last for hundreds of picoseconds. Such correlations are not observed in the random walk models of noninteracting particles. It is suggested that the observed large-scale correlations reveal the vortex-like motions of the molecules.

Argon and water

Computer simulation of liquid argon and water has shown that the structural inhomogeneity pattern visualized by coloring their instantaneous structures according to the volumes of the Voronoi polyhedra is the same for both liquids. The dynamics of argon atoms in the cavity of a clathrate hydrate and in liquid water is considered. The argon atoms included in the network of hydrogen bonds in liquid water are involved in concerted motion. As a result of this concerted motion, the distance between two argon atoms can fluctuate around some value for a fairly long time. The environment of argon atoms in a liquid aqueous solution differs significantly from their environment in clathrate hydrates.

Collective Effects in Molecular Motions in Liquids

The collective effects in water were studied by investigating the spatial distribution of long-living hydrogen bonds and revealing correlations in molecular motions. The existence of extended clusters, whose molecules are linked by long-living bonds, suggests the existence of correlations between the motions of its molecules. The mean scalar products of the shift vectors of two molecules were calculated using the narrow ranges (DP) of intermolecular distances in the initial configuration. The average correlation coefficients (the cosines of angles between the shift vectors of two molecules) were also calculated. The DP and cosine values were averaged over all pairs with this intermolecular distance. The DP values increased with time and formed a plateau after a few hundred picoseconds. The plateau was attributed to the existence of molecular vortices that cover large (several nanometers) volumes of the liquid. The conclusion was drawn that hydrophobic species, for example, noble gas atoms incorporated in the water net could be involved in collective motions.

Structural Nonhomogeneity of Low-Density Amorphous Ice and Its Effect on the Dynamics of Water Molecules

This paper reports on a molecular dynamics simulation of low-density (0.94 g/cm3) amorphous ice using Poltev–Malenkovs potential (729 particles, 101 K). The model is characterized by structural nonhomogeneity, manifesting itself as nonrandom arrangement of molecules with different nearest environments (different tetrahedricities of the nearest environment and different Voronoi polyhedron volumes). The nonhomogeneity regions are fractal, their typical size being ∼1 nm. The dynamic characteristics of the water molecules (motion amplitudes in a cage and vibrational spectra) differ from one structural region to another.

Long-time correlations in the diffusive motion of liquid argon atoms

The dynamics of liquid argon (∼49 000 atoms in a cubic periodic cell) near the triple point is simulated. Two-particle correlation functions describing the simultaneous diffusive displacement of a pair of particles are calculated. Time dependence of the mean scalar product of the displacement vectors of two particles reaches a plateau. Previously, we detected such a behavior of this function for liquid water.

Structural Inhomogeneity of High‐Density Amorphous Ice

Modeling of high‐density amorphous ice (1.17 g/cm3) was carried out at ∼100 K (576 species) by a molecular‐dynamics method using the Poltev–Malenkov potential. The revealed structural inhomogeneity of the model is similar to that established earlier for water and low‐density amorphous ice. Different structural zones (i.e., zones with different degrees of deviation of the configuration of the nearest neighbor environment from a regular tetrahedron and zones with different volumes of Voronoi polyhedra) show different dynamic characteristics of water molecules (mean square of displacement of the center of mass and vibration spectrum).

Helium, neon, and water

The molecular dynamics method is used to study liquid aqueous solutions of helium and neon, liquid water films and solid films with an ice II structure in helium and neon atmospheres, and solid solutions of helium and neon in ice II. Gas atoms wander randomly in water and make occasional slow jumps. The structure of the hydrate shell of the gas atoms bears little resemblance to the structure of ice II and other ice modifications. The solubility of neon in a water film is only a little higher than that of helium. Helium and neon atoms that find themselves in the channels of a thin ice II film make the same jumps along the channels as those along the channels in the structure of ice II crystals. The motions of two neon atoms in the neighboring (along the z axis) planes are correlated, whereas there is no correlation between the motions of helium atoms.

Visualization of collective vortex-like motions in a computer model of liquid argon

Visualization pictures of collective motion of particles are presented, showing the existence of mesoscopic (of the order of tens of angstroms) vortex-like motions at time intervals of at least hundreds of picoseconds in molecular dynamics models of liquid argon.

Coherent molecular motion in aquatic environment. Extraction of correlation from noise

Molecular dynamics simulation of liquid water and system “argon-thin water film” has been performed. In was shown that time dependence of distances between oxygen atoms of water molecules, connected by hydrogen bonds, looks like random noise and these molecules are doomed to move concertedly. The average distance between the argon atoms dissolved in water is retained for some time, so the argon atoms included in the hydrogen bond networks, but not capable to participate in hydrogen bonds are involved in concerted motions. The problem of revealing the correlations in the motion of molecules and atoms is discussed.