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Methanol as a basic liquid and the simplest alcohol is widely used in industry and scientific experiments. However, there are still no reliable data on the thermodynamic properties of methanol at high pressure. Here, we present an experimental and computational study of the thermodynamic properties of liquid methanol under high pressure up to 15 kbar, which significantly exceeds previously reported pressures. A temperature response to a small adiabatic change in pressure has been measured using a piston–cylinder apparatus. We have compared our experimental results with the literature data for lower pressures and NIST approximations. We find that all existing experimental data do not agree with each other and with our experiments. The NIST approximations are mainly based on low pressure data and appear to be unreliable in the high pressure region, giving even qualitatively wrong results. OPLS and COMPASS force field models have been used in the method of molecular dynamics. The agreement of molecular simulation with our experimental data is definitely unsatisfactory, which means that the most common computational models of methanol are not sufficiently good. We hope that these experimental data and approximations will help in developing better computational models.

The structure of a simple monatomic system in a slit pore has been studied by means of the molecular dynamics simulation. Nowadays, it is supposed that the structure of crystals in a narrow pore (narrower than approximately eight atomic diameters) should be described as a set of two-dimensional layers with either triangular, or square symmetry. In addition, a buckled phase can appear with particles located in zigzag way around a plane. It is shown that all these structures can be considered as cuts of FCC or HCP structures.

The paper discusses current theories of melting of two-dimensional systems that are mainly based on the concepts of the Berezinskii–Kosterlitz–Thouless (BKT) theory of phase transitions in two-dimensional systems with continuous symmetry. At present there are three basic scenarios of melting of two-dimensional crystals that as shown by computer simulation are implemented in the two-dimensional system of Hertzian spheres used to describe the behavior of a number of soft matter systems. The system demonstrates complex phase behavior with the presence of all three melting scenarios in the various parts of the phase diagram.

Strontium chloride is a well-known crystalline superionic conductor. Because of this, a lot of attention was paid to its properties in crystalline phase, especially in the vicinity of transition in...

We report ulsrasonic measurements of the density and bulk modulus of propylene glycol at room temperature and at the temperature of liquid nitrogen up to a pressure of 1.4 GPa. We perform molecular dynamics simulations with two different force fields—COMPASS and Charmm. We show that the COMPASS force field reproduces the experimental data with high accuracy. For this reason we employ this force field to analyze the microstructure and hydrogen bonding in the system. We find that the number of hydrogen bonds does not depend on pressure.

The anomalous behavior of a two-dimensional system of Hertzian disks with exponent α=7/2 has been studied using the method of molecular dynamics. The phase diagram of this system is the melting line of a triangular crystal with several maxima and minima. Waterlike density and diffusion anomalies have been found in the reentrant melting regions. Noteworthy, a density anomaly has been observed not only in the liquid and hexatic but also in the solid phase. The calculations of the phonon spectra of longitudinal and transverse modes have yielded negative dependence of the frequency of transverse modes on density along all directions in the regions with a density anomaly. This indicates an association of the density anomaly with transverse oscillations of the crystal lattice. The regions of density and diffusion anomalies have been drawn on the phase diagram. It has been found that the stability regions of anomalous diffusion extend to temperatures well above the maximum melting point T=0.0058 of the triangular crystal.

The melting lines of graphite and liquid carbon have been studied for a long time. However, numerous controversies still remain in this field; for instance, different experiments give different melting temperatures. In this work, we explore the melting lines of graphite and graphene by means of classical and ab-initio molecular dynamics. We show that the empirical models developed on the basis of experimental data (AIREBO potential, Tersoff potential and some others) fail to reproduce the properties of liquid carbon properly. The models fitted to ab-initio data (LCBOPII and GAP) give much better results. However, both types of empirical models and ab-initio simulations evidence the presence of smooth structural crossover in liquid carbon. We also show that the “melting” of graphene discussed in previous works on computer simulation is indeed sublimation and propose a novel method to simulate the melting of graphene: simulation of graphene sheet in argon atmosphere. The melting temperature of graphene in argon atmosphere appears to be close to the melting temperature of graphite.

Solid structures with different types of ordering (crystals, random tilings and glasses) are considered. We show that the short-range order structure of random tilings characterised by radial distr...

We present the results of a computer simulation study of the longitudinal dispersion curves of a model core-softened liquid and the SPC/E water model. We have shown that the excitation frequencies decreased with temperature along isochors in both systems. This behavior is contrary to the normal liquids. We have introduced new water anomaly: anomalous dependence of excitation frequencies on temperature.

Using molecular dynamics simulation we have investigated the influence of random pinning on the phase diagram and melting scenarios of a two-dimensional system with the Hertz potential for $\ensuremath{\alpha}=5/2$. It has been shown that random pinning can cardinally change the mechanism of first-order transition between the different crystalline phases (triangular and square) by virtue of generating hexatic and tetratic phases: a triangular crystal to hexatic transition is of the continuous Berezinskii-Kosterlitz-Thouless (BKT) type, a hexatic to tetratic transition is of first order, and finally, there is a continuous BKT-type transition from tetratic to the square crystal.

Heat capacity of several liquids which demonstrate anomalous behaviour is studied. It is shown that isochoric heat capacity of these liquids shows anomalously high values which is related to smooth...

A soft-sphere system is one of the simplest and most studied models of many-body problems. In contrast to real substances, it does not have a cohesive state and boiling line. For this reason, the fluid phase has been considered to be the same everywhere on the phase diagram with no difference between the gas and liquid phase. Here, we show that the soft-sphere fluid can in fact be demarcated into two states with liquid-like and gas-like properties even though these states are not separated by a thermodynamic phase transition. We find that particle dynamics and excitation spectra are qualitatively different in the two states. This difference applies to other system properties such as viscosity, diffusion and speed of sound. We consider different repulsion exponents of the soft-sphere system as well as the limiting case of the hard-sphere system and comment on their implications for demarcating the fluid phase into liquid-like and gas-like states.

We present computer simulations of a system of purely repulsive soft colloidal particles interacting via the Hertz potential and constrained to a two-dimensional plane. This potential describes the elastic interaction of weakly deformable bodies and can be a reliable model for qualitative description of behavior of soft macromolecules, like globular micelles and star polymers. We find a large number of ordered phases, including the dodecagonal quasicrystal, and analyze the melting scenarios of low density triangle and square phases. It is interesting that depending on the position on the phase diagram the system can melt both through the first order transition and in accordance with the Berezinskii-Kosterlitz-Thouless-Halperin-Nelson-Young (BKTHNY) scenario (two continuous transitions with the intermediate hexatic phase) and also in accordance with recently proposed two-stage melting with the first order hexatic-isotropic liquid transition and continuous solid-hexatic transition. We also demonstrate the possibility of the tricritical point on the melting line., 11 pages, 18 figures

A transition from a square to a hexagonal lattice is studied in a 2D system of particles interacting via a core-softened potential. Due to the presence of two length scales of repulsion, different local configurations with four, five, and six neighbors are possible, leading to the formation of complex crystals. The previously proposed interpolation method is generalized to calculate pair correlations in crystals whose unit cell consists of more than one particle. The high efficiency of the method is illustrated using a snub square lattice as a representative example. Molecular dynamics simulations show that the snub square lattice is broken upon heating, generating a high-density quasicrystalline phase with 12-fold symmetry (HD12 phase). A simple theoretical model is proposed to explain the physical mechanism responsible for this phenomenon: with an increase in the density (from square to hexagonal phases), the concentrations of different local configurations randomly realized through a plane tiling change, which minimizes the energy of the system. The calculated phase diagram in the intermediate density range justifies the existence of the HD12 phase and demonstrates a cascade of first-order transitions "square - HD12 - hexagonal" solid phases with increasing density. The results allow us to better understand the physical mechanisms responsible for the formation of quasicrystals, and, therefore, should be of interest for broad community in materials science and soft matter.

The representation of the Widom line as a line of maximums of the correlation length and a whole set of thermodynamic response functions above the critical point were introduced to describe anomalies observed in water above the hypothetical critical point of the liquid-liquid transition. The supercritical region for the gas-liquid transition was also described later in terms of the Widom line. It is natural to assume that an analogue of the Widom line also exists in the supercritical region for the first-order isostructural transition in crystals, which ends at a critical point. We use a simple semiphenomenological model, close in spirit the van der Waals theory, to study the properties of the new Widom line. We calculate the thermodynamic response functions above the critical point of the isostructural transition and find their maximums determining the Widom line position.

Using molecular simulation, we studied the spectra of longitudinal and transverse excitations of water (the SPC/E model) and simple liquid (the Lennard-Jones system) along isobars. Comparison of the results for water and simple liquid has shown that the frequency of longitudinal excitations of water demonstrates strong temperature dependence at low temperatures and comes to saturation at high ones. The low temperature regime is in qualitative agreement with the published experimental spectra of longitudinal excitations of water along an isobar. The high temperature regime has not been studied experimentally up to now.

Heat capacity is among the most important thermodynamic characteristics of a substance. The behaviour of the heat capacity is well understood in gases and crystals, but not in liquids. A common view on the heat capacity of liquids is that it should be close to the solid-like values close to the melting line and it should approach the gas values in the limit of high temperatures. However, some liquids can show anomalously high magnitudes of isochoric heat capacity. In the present paper, I show that core-softened liquids can demonstrate unusually high magnitude of the heat capacity induced by a structural crossover of the liquid.

We consider the melting of a two-dimensional system of collapsing hard disks (a system with a hard-disk potential to which a repulsive step is added) for different values of the repulsive-step width. We calculate the system phase diagram by the method of the density functional in crystallization theory using equations of the Berezinskii–Kosterlitz–Thouless–Halperin–Nelson–Young theory to determine the lines of stability with respect to the dissociation of dislocation pairs, which corresponds to the continuous transition from the solid to the hexatic phase. We show that the crystal phase can melt via a continuous transition at low densities (the transition to the hexatic phase) with a subsequent transition from the hexatic phase to the isotropic liquid and via a first-order transition. Using the solution of renormalization group equations with the presence of singular defects (dislocations) in the system taken into account, we consider the influence of the renormalization of the elastic moduli on the form of the phase diagram.

In the present paper we study the thermodynamic properties of superionic conductor $SrCl_2$ at high temperatures by means of molecular dynamics method. Firstly, we calculate the melting line. Then we compute the equations of state and the response functions (heat capacity, thermal expansion coefficient, etc) at the temperatures up to the melting. We show that the response functions show maxima or minima at the temperatures well above the temperature of transition into the conductive state, and therefore are not related to this transition.

Investigation of the longitudinal and transverse excitations in liquids is of great importance for understanding the fundamentals of the liquid state of matter. One of the important questions is the temperature and density dependence of the frequency of the excitations. In our recent works it was shown that while in simple liquids the frequency of longitudinal excitations increases when the temperature is increased isochorically, in water the frequency can anomalously decrease with the temperature increase. In the present manuscript we study the dispersion curves of longitudinal and transverse excitations of water and liquid silicon modelled by Stillinger-Weber (SW) potential. We show that both in liquid silicon and SW model of water the frequencies of longitudinal excitations slightly increase with temperature which is in contrast to the results for SPC/E model of water.

We present the results of a computer simulation study of a finite temperature phase diagram of two-dimensional and quasi two-dimensional core-softened systems both taking into account long-range Coulomb-like forces and ignoring them. The system structure was determined from analysis of the behavior of radial distribution functions, order parameters and number of nearest neighbors. The system has been shown to have a large number of different phases. We have found that long-range forces substantially affected the structure and the melting point of the system at low and moderate densities, while at high densities the effect of long-range forces was negligible.

The phase diagram of a two-dimensional system with purely repulsive monotonic potential is studied by means of molecular dynamics method. It is shown that the system demonstrates a set of ordered phases, including such phases as dimers, oblique crystal and Kagome phase. The regions of stability of all ordered phases in the system is calculated. The scenarios of melting of triangular and square crystals of the system are identified.

We present the molecular simulation study of the behaviour of water and sodium chloride solution confined in lizardite asbestos nanotube which is a typical example of hydrophilic confinement. The local structure and orientational and dynamic properties are studied. It is shown that at low enough temperatures there is a well-defined orientational ordering of the water molecules. At high local densities corresponding to the maxima of the density distribution function, the water molecules are oriented parallel to the axis of the tube. It is also shown that the diffusion coefficient drops about two orders of magnitude comparing to the bulk case. The behaviour of sodium chloride solutions is also considered and the formation of double layer is observed.

Water is the most common liquid on the Earth. At the same time, it is the strangest liquid having numerous anomalous properties. For this reason, although water was investigated in numerous studies, many questions still remain unanswered. Even the thermodynamic properties of water at high pressures are unknown. In this paper, we present an experimental study of the thermodynamic properties of water up to a pressure of 12 kbar and a temperature of 473 K far above the range of pressures and temperatures in previous studies. We compare the experimental results to the results of computer simulations of two models of water (SPC/E and TIP4P) and show that the SPC/E model is not appropriate at high pressure, while the TIP4P model describes the equation of state of water, but fails to describe the heat capacity.

It has been recently proposed that supercritical fluids experience a crossover between two different regimes of the particle dynamics. The crossover takes place at the Frenkel line which can be defined from vanishing of oscillations of the velocity autocorrelation functions (vacfs). Here, we present a novel detailed analysis of vacfs of binary isotopic mixtures of Lennard-Jones and Soft Spheres particles and consider temperature dependence of vacfs of individual components as well as mixtures. We show that partial vacfs can be very different: for the mass ratio of about 2, the temperature at which the oscillatory behavior of vacf vanishes differs eightfold. Notably, oscillatory behavior of vacfs of the whole mixture vanishes at about the same temperature as in the one-component fluid. We conclude that the criterion to locate the Frenkel line on the basis of vacfs is general and applies to mixtures, reflecting more fundamental changes of system dynamics such as the disappearance of transverse mode from the excitation spectrum.

We review recent work aimed at understanding dynamical and thermodynamic properties of liquids and supercritical fluids. The focus of our discussion is on solid-like transverse collective modes, whose evolution in the supercritical fluids enables one to discuss the main properties of the Frenkel line separating rigid liquid-like and non-rigid gas-like supercritical states. We subsequently present recent experimental evidence of the Frenkel line showing that structural and dynamical crossovers are seen at a pressure and temperature corresponding to the line as predicted by theory and modelling. Finally, we link dynamical and thermodynamic properties of liquids and supercritical fluids by the new calculation of liquid energy governed by the evolution of solid-like transverse modes. The disappearance of those modes at high temperature results in the observed decrease of heat capacity.

In the present paper, we discuss the so-called stripe phase of two-dimensional systems observed in computer simulation of a core-softened system and in some experiments with colloidal films. We have shown that stripe phase is actually an oblique crystal and found out its unit vectors, i.e. we give a full description of the structure of this crystalline phase.

We present the results of a computer simulation study of the melting scenario of two-dimensional soft-disk systems with potential $U(r)=\varepsilon(\sigma/r)^n$, $n=12$ and $n=1024$, both in the presence of random pinning and without it. The melting parameters have been determined from analysis of the behavior of equations of state, correlation functions of the orientational and translational order parameters, Young modulus and renormalization group equations. The transition points obtained from these criteria are in good agreement. We have shown that the systems melted in two stages - first-order hexatic phase to liquid transition and the continuous Berezinskii-Kosterlitz-Thouless type crystal-hexatic phase transition. Random pinning widened the hexatic phase, but left the melting scenario unchanged., Comment: 9 pages, 12 figures

We study thermo-mechanical properties of matter at extreme conditions deep in the supercritical state, at temperatures exceeding the critical one up to four orders of magnitude. We calculate the Gr\"{u}neisen parameter {\gamma} and find that it decreases with temperature from 3 to 1 on isochores depending on the density. Our results indicate that from the perspective of thermo-mechanical properties, the supercritical state is characterized by the wide range of {\gamma} which includes the solid-like values - an interesting finding in view of the common perception of the supercritical state as being an intermediate state between gases and liquids. We rationalize this result by considering the relative weights of oscillatory and diffusive components of the supercritical system below the Frenkel line. We also find that {\gamma} is nearly constant at the Frenkel line above the critical point and explain this universality in terms of pressure and temperature scaling of system properties along the lines where particle dynamics changes qualitatively.

We develop an approach to liquid thermodynamics based on collective modes. We perform extensive molecular dynamics simulations of noble, molecular and metallic liquids and provide the direct evidence that liquid energy and specific heat are well-described by the temperature dependence of the Frenkel (hopping) frequency. The agreement between predicted and calculated thermodynamic properties is seen in the notably wide range of temperature spanning tens of thousands of Kelvin. The range includes both subcritical liquids and supercritical fluids. We discuss the structural crossover and inter-relationships between structure, dynamics and thermodynamics of liquids and supercritical fluids., Comment: 10 pages, 6 figures

At present, supercritical technologies use fluids (for example, CO2, H2O) at temperatures and pressures close to critical. It is not currently clear how important critical fluctuations are for dissolution processes and chemical reactions in supercritical fluids. At the same time, our recent work has shown that qualitative changes in the system behavior occurred also at temperatures and pressures much higher than critical. This paper briefly summarizes the latest results of the search and study of the parameters P and T of the changes of dynamic types in supercritical fluids. It turned out that this region of parameters corresponded to a narrow band, namely, the “Frenkel line.” Reaching this line with increasing temperature corresponds to the disappearance of shear excitations in the liquid at all frequencies. The Frenkel line can be considered as a boundary between the liquid and the dense gas at extremely high pressures far from the critical point. Recently, a simple and mathematically rigorous method for finding the Frenkel line was proposed based on analysis of the autocorrelation function of particle velocities. We suggest using the data of the Frenkel line parameters for actual fluids to move supercritical technologies into the region of extremely high pressures exceeding critical ones by tens and hundreds of times.

We have studied the dependence of the excitation frequency of water along an isochore and an isotherm crossing the region of density anomaly. We have shown that the frequency of longitudinal excitations demonstrated anomalous temperature dependence along the isochore. At the same time the dependence of both longitudinal and transverse excitation frequencies on the density along the isotherm was very modest or even negligible in a fairly wide range of densities. This kind of behavior also seems anomalous in comparison with ordinary liquids.

The present article presents a molecular dynamics study of several anomalies of core-softened systems. It is well known that many core-softened liquids demonstrate diffusion anomaly. Usual intuition relates the diffusion coefficient to shear viscosity via Stockes-Einstein relation. However, it can break down at low temperature. In this respect it is important to see if viscosity also demonstrates anomalous behavior., Comment: 7 pages, 9 Figures

Despite about forty years of investigations, the nature of the melting transition in two dimensions is not completely clear. In the framework of the most popular Berezinskii-Kosterlitz-Thouless-Halperin-Nelson-Young (BKTHNY) theory, 2D systems melt through two continuous Berezinskii-Kosterlitz-Thouless (BKT) transitions with intermediate hexatic phase. The conventional first-order transition is also possible. On the other hand, recently on the basis of computer simulations the new melting scenario was proposed with continuous BKT type solid-hexatic transition and first order hexatic-liquid transition. However, in the simulations the hexatic phase is extremely narrow that makes its study difficult. In the present paper, we propose to apply the random pinning to investigate the hexatic phase in more detail. The results of molecular dynamics simulations of two dimensional system having core-softened potentials with narrow repulsive step which is similar to the soft disk system are outlined. The system has a small fraction of pinned particles giving quenched disorder. Random pinning widens the hexatic phase without changing the melting scenario and gives the possibility to study the behavior of the diffusivity and order parameters in the vicinity of the melting transition and inside the hexatic phase., 7 pages, 6 figures. arXiv admin note: substantial text overlap with arXiv:1608.05232

In this paper we present a simulation study of water-like anomalies in a purely repulsive core-softened system and in a system with attraction described in our previous publications. We investigate the anomalous regions for systems with the same functional form of the potentials but with different parameters and show that the order of the region of anomalous diffusion and the region of density anomaly is inverted with increasing the width of the repulsive shoulder and the depth of the attractive well. It is shown that while the density anomaly is always inside the region of the structural anomaly, the diffusion anomaly can change its location depending on the parameters of the potential. In the presence of the attraction in the potential, the system demonstrates the silica-like behavior.

Molecular dynamics computer simulation has been used to compute in wide pressure-temperature region the self-diffusion coefficient and shear viscosity of soft-sphere fluids, in which the particles interact through the inverse power pair potential. The limitations of the previous empirical approaches and models for the description of the kinetic coefficients are shown. On the basis of the simulations the universal functions for diffusion coefficient and shear viscosity of soft spheres are constructed which allow to compute these quantities for any density and temperature. We show that the behavior of shear viscosity at high temperatures is more complex than the one of diffusion coefficient and give quantitative explanation of this result.

The locus of extrema (ridges) for heat capacity, thermal expansion coefficient, compressibility, and density fluctuations for model particle systems with Lennard-Jones (LJ) potential in the supercritical region have been obtained. It was found that the ridges for different thermodynamic values virtually merge into a single Widom line at T < 1.1T(c) and P < 1.5P(c) and become practically completely smeared at T < 2.5T(c) and P < 10P(c), where T(c) and P(c) are the critical temperature and pressure. The ridge for heat capacity approaches close to critical isochore, whereas the lines of extrema for other values correspond to density decrease. The lines corresponding to the supercritical maxima for argon and neon are in good agreement with the computer simulation data for LJ fluid. The behavior of the ridges for LJ fluid, in turn, is close to that for the supercritical van der Waals fluid, which is indicative of a fairly universal behavior of the Widom line for a liquid-gas transition.

In the mode-coupling approximation, we consider the transition to the glass state in a system of collapsing hard spheres (a system with the hard-core potential to which a repulsive step is added). We propose an approximation for the structure factor of the system, which we use to construct the phase diagram of the transition to the glass state. We show that there exists a maximum on the liquid-glass curve corresponding to the reentrant transition to the glass state in the system. In the framework of the proposed model, we consider bifurcations of solutions of the equations describing the transition to the glass state and show that there exist bifurcations of the “swallow-tail” type corresponding to the glass-glass transition.

The comprehensive computer simulation study of the phase diagram of the repulsive step potential system in three dimensions is represented. We show that the system with a simple purely repulsive isotropic potential demonstrates a number of unusual features. The maxima and minima on the melting curve are found for some regions of potential parameters. It is shown that the phase diagram in ρ-T plane includes two isostructural crystalline parts separated by the disordered phase which is amorphous at low enough temperatures. Phase diagram in the (P-T) plane shows that the transition to the amorphous state occurs approximately along the extrapolated spinodals. Structural FCC-BCC phase transition is found at high densities.

Recent research suggests that the supercritical state consists of liquidlike and gaslike states where particle dynamics and key system properties are qualitatively different. We report experimental evidence of the structural crossover in supercritical neon at pressure and temperature conditions significantly exceeding the critical point values: 250$\,P_c$ and 6.6$\,T_c$. The experimental results show a crossover of the medium-range order structure evidenced by the change of the structure factor with pressure. We also observe the crossover of the short-range order structure indicated by changes in the coordination number. The relative width of the crossover is fairly narrow and is smaller than 10-12 \% in pressure and temperature. By comparing our experimental results with molecular dynamics simulations, we suggest that the observed crossover can be attributed to the Frenkel line and discuss the relationship between the structural crossover and qualitative changes of dynamical and thermodynamic properties of supercritical matter., Comment: 7 pages, 6 figures

It is well known that confining a liquid into a pore strongly alters the liquid behavior. Investigations of the effect of confinement are of great importance for many scientific and technological applications. Here we present a molecular dynamics study of the behavior of cyclohexane confined in carbon slit pores. The local structure and orientational ordering of cyclohexane molecules are investigated. It is shown that the system freezes with decreasing the pore width, and the freezing temperature of nanoconfined cyclohexane is higher than the bulk one.

Dynamical crossover in water is studied by means of computer simulation. The crossover temperature is calculated from the behavior of velocity autocorrelation functions. The results are compared with experimental data. It is shown that the qualitative behavior of the dynamical crossover line is similar to the melting curve behavior. Importantly, the crossover line belongs to experimentally achievable (P, T) region which stimulates the experimental investigation in this field.