Development of a united-atom force field for 1-ethyl-3-methylimidazolium tetracyanoborate ionic liquid

Three united-atom (UA) force fields are presented for the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate, abbreviated as [EMIM]⁺[B(CN)₄]^?. The atomistic charges were calculated based on the restrained electrostatic potential (RESP) of the isolated ions (abbreviated as force field 1, FF-1) and the ensemble averaged RESP (EA-RESP) method from the most stable ion pair configurations obtained by MP2/6-31G*+ calculations (abbreviated as FF-2 and FF-3). Non-electrostatic parameters for both ions were taken from the literature and Lennard–Jones parameters for the [B(CN)₄]^? anion were fitted in two different ways to reproduce the experimental liquid density. Molecular dynamics (MD) simulations were performed over a wide temperature range to identify the effect of the electrostatic and non-electrostatic potential on the liquid density and on transport properties such as self-diffusion coefficient and viscosity. Predicted liquid densities for the three parameter sets deviate less than 0.5% from experimental data. The molecular mobility with FF-2 and FF-3 using reduced charge sets is appreciably faster than that obtained with FF-1. FF-3 presents a refined non-electrostatic potential that leads to a notable improvement in both transport properties when compared to experimental data.     

Pressure-induced phase transition and structural properties of CrO2

The structural properties and pressure-induced phase transitions of CrO₂ have been investigated using the pseudopotential plane-wave method based on the density functional theory (DFT). The rutile-type (P4₂/mnm), CaCl₂-type (Pnnm), pyrite-type (Pā3), and CaF₂-type (Fm-3m) phases of CrO₂ have been considered. The structural properties such as lattice parameters, bulk moduli and its pressure derivative are consistent with the available experimental data. The second-order phase-transition pressure of CrO₂ from the rutile phase to CaCl₂ phase is 10.9?GPa, which is in good agreement with the experimental result. The sequence of these phases is rutile-type?→?CaCl₂-type?→?pyrite-type?→?CaF₂-type with the phase-transition pressures 10.9, 23.9, and 144.5?GPa, respectively. The equation of state of different phases has also been presented. It is more difficult to compress with the increase of pressure for different phases of CrO₂.     

An effective force field for molecular dynamics simulations of dimethyl sulfone

A rigid five-site united atom model for dimethyl sulfone (DMSO₂) compatible with the GROMOS force field is parametrized and tested. The parameters were optimized with respect to experimental quantities such as liquid density, heat of vaporization, shear viscosity and excess free energy. Good agreement with pure component properties is achieved except for the static dielectric permittivity which is calculated too low. Together with the SPC model for water the new DMSO₂ model was used to study aqueous mixtures at low concentrations and compared to aqueous mixtures of DMSO. It is concluded that interaction parameters for sulfoxide oxygen are not directly transferable to sulfonyl oxygen.     

A classical density functional approach to depletion interaction of Lennard-Jones binary mixtures

In this article, we apply classical density functional theory to investigate the characteristics of depletion interaction in Lennard-Jones (LJ) binary fluid mixtures. First, to confirm the validity of our adopted density functional formalism, we calculate the radial distribution functions using a theoretical approach and compare them with results obtained by molecular dynamics simulation. Then, this approach is applied to two colloids immersed in LJ solvent systems. We investigate the variation of depletion interaction with respect to the distance of two colloids in LJ binary systems. We find that depletion interaction may be attractive or repulsive, mostly depending on the bulk density of the solvent and the temperature of the binary system. For high bulk densities, the repulsive barrier of depletion force is remarkable when the total excluded volume of colloids touches each other and reaches a maximum. The height of the repulsive barrier is related to the parameters of the LJ potential and bulk density. Moreover, the depletion force may exhibit attractive wells if the bulk density of the solvent is low. The attractive well tends to appear when the surface–surface distance of colloids is half of the size of the polymer and deepens with temperature lowering in a fixed bulk density. In contrast with the hard-sphere system, no oscillation of depletion potential around zero is observed.     

Ground state and phonon spectrum of NiSi2

We have studied structural, electronic, elastic and dynamical properties of NiSi₂ by employing the plane wave pseudopotential method based on density functional theory within the local density approximation. The calculated lattice constant, bulk modulus and first-order pressure derivative of the bulk modulus are reported and compared with earlier available experimental and theoretical calculations. Numerical first-principles calculations of the elastic constants were used to calculate C₁₁, C₁₂ and C₄₄ for NiSi₂. The calculated electronic band structure has been compared with angle-resolved photoemission spectroscopy experimental data along the [100] and [111] symmetry directions. A linear response approach to density functional theory is used to derive the phonon dispersion curves and phonon partial density of states. Atomic displacement patterns for NiSi₂ at the Γ, X and L symmetry points are also presented.     

Ab initio investigation of the structural,elastic and electronic properties of the anti-perovskite TlNCa3

We have investigated the structural, elastic and electronic properties of the anti-perovskite TlNCa₃ using ab initio calculations within the generalized gradient approximation and the local density approximation for the exchange–correlation potential. The lattice constant, bulk modulus, elastic constants and their pressure dependence, energy band structures, density of states and charge density distribution are calculated and analyzed in comparison with the available experimental and theoretical data. The bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, Lamé’s coefficients, average sound velocity and Debye temperature are numerically estimated for ideal polycrystalline TlNCa₃ aggregates in the framework of the Voigt–Reuss–Hill approximation. This is the first theoretical prediction of the elastic constants and their related properties for TlNCa₃ that requires experimental confirmation.     

Viscosity of Collagen Solutions: Influence of Concentration,Temperature, Adsorption,and Role of Intermolecular Interactions

An investigation of the influences of the adsorption, electrolyte, shear rate, and temperature on the viscometric behavior of collagen solutions was performed using a photoelectric viscometer. The experimental results showed an abnormal behavior of the reduced viscosity (η_sp/C) of dilute collagen solutions measured by a viscometer with or without a hydrophobic surface treatment; the reduced viscosity increased with decreasing concentration. This phenomenon can be completely eliminated by increasing the concentration of an added salt. It indicated that the abnormal viscosity resulted from a long-ranged, inter-molecular electrostatic force. The reduced viscosity decreased as the shear rate increased. The shear-thinning cavitation phenomenon resulted from the high aspect ratio of the collagen molecules. The reduced viscosity of the collagen solutions increased with an increase in temperature, which was due to the association of collagen molecules at high temperature.     

First-principles investigations on elastic, phonon and thermodynamic properties of SrB6 under pressure

The elastic, phonon and thermodynamic properties of the divalent alkaline-earth hexaboride SrB₆ are investigated by using plane-wave pseudopotential density functional theory method. The calculated structure parameters and bulk modulus are well consistent with the available experiment and theoretical data. The pressure dependences of elastic constants C_ij, bulk modulus B₀, shear modulus G, Young's modulus E and Poisson's ratio σ are also presented. With these elastic parameters, we investigate the mechanical stability and compressibility of SrB₆. For the thermodynamic properties, both phonon and quasi-harmonic Debye model methods are adopted. Through the comparison with experimental and other theoretical results, we found the method of quasi-harmonic Debye model is a little better. Moreover, the phonon dispersion relations are also obtained. It is found that there are two LO/TO splitting around 5 THz and 26 THz, respectively.     

Structural,elastic and thermodynamic properties of tetragonal and orthorhombic polymorphs of Sr2GeN2: an ab initio investigation

The structural, elastic and thermodynamic properties of the α (tetragonal) and β (orthorhombic) polymorphs of the Sr₂GeN₂ compound have been examined in detail using ab initio density functional theory pseudopotential plane-wave calculations. Apart the structural properties at the ambient conditions, all present reported results are predicted for the first time. The calculated equilibrium lattice parameters and inter-atomic bond-lengths of the considered polymorphs are in good agreement with the available experimental data. It is found that α-Sr₂GeN₂ is energetically more stable than β-Sr₂GeN₂. The two examined polymorphs are very similar in their crystal structures and have almost identical local environments. The single-crystal and polycrystalline elastic parameters and related properties – including elastic constants, bulk, shear and Young’s moduli, Poisson’s ratio, anisotropy indexes, Pugh’s criterion, elastic wave velocities and Debye temperature – have been predicted. Temperature and pressure dependence of some macroscopic properties – including the unit-cell volume, bulk modulus, volume thermal expansion coefficient, heat capacity and Debye temperature – have been evaluated using ab initio calculations combined with the quasi-harmonic Debye model.     

Effects of magnesium salt concentrations on B-DNA overstretching transition

In this study, we use optical tweezers to investigate the ionic effects of magnesium salt solutions on the overstretching transition of single B-DNA molecules. The experimental data are compared with those in sodium salt solutions. The overstretching transition force increases when the NaCl or MgCl₂ salt concentration increases. Magnesium cations have much stronger effects on the overstretching transition force than sodium cations. For both NaCl and MgCl₂ salt solutions, the overstretching transition force is linear with the natural logarithm of salt concentration, which confirms the theory proposed in previous paper. The modified ZZO model is applied to study the electrostatic contribution of magnesium salt solutions to the overstretching transition of single B-DNA molecules. The consistency between the experimental data and analytical results shows that the modified ZZO model can simulate the transition behavior of single B-DNA molecules in different NaCl and MgCl₂ salt solutions.     

Calculation of thermophysical properties for CO2 gas using an ab initio potential model

The density, isochoric heat capacity, shear viscosity and thermal conductivity of CO₂ gas in the pressure range of 1–50 atm and 300 K are calculated based on a five-centre potential model obtained from ab initio calculations of the intermolecular potential of a CO₂ dimer. The quantum effects of the intramolecular motion are included in a model by the Monte Carlo (MC) Method. Without using any experimental data, the present model achieves excellent agreements between the calculated thermophysical properties and experimental data for all simulated CO₂ densities except the highest one at 135 kg/m³ (3 mol/L). The contributions of potential to the thermophysical properties of the moderate dense CO₂ gas and their dependence on density are investigated in detail.     

Second-order and third-order elastic constants of B4C ceramics

The linear and nonlinear elastic properties of B₄C boron carbide ceramics have been studied. The second-order elastic constants and other parameters of the theory of elasticity in the linear approximation have been calculated based on the experimentally measured density and velocity values of longitudinal and shear bulk acoustic waves in the samples. The Thurston-Brugger method has been used to determine the third-order elastic constants of B₄C. To achieve this, we have measured the relative changes of the longitudinal and shear bulk acoustic wave velocities depending on the uniaxial compression applied to the sample.     

Stress relaxation and viscosity of a bulk Pd<Subscript>40</Subscript>Cu<Subscript>30</Subscript>Ni<Subscript>10</Subscript>P<Subscript>20</Subscript> metallic glass under isochronous heating conditions

Isochronous relaxation of tensile stresses is measured in a bulk Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glass in the initial state and after certain thermal treatments. The results of measurements are used to find the energy spectrum of irreversible structural relaxation, from which the temperature dependence of shear viscosity is then calculated. This dependence is also found independently from measurements of creep in the same glass. The calculated viscosity is shown to agree well with the experimental data.     

Bulk Viscous Bianchi Type I Cosmological Models with Time-Dependent Cosmological Term Λ

Bianchi type I cosmological models with time-varying cosmological constant Λ and bulk viscous fluid are investigated. Cosmic matter is chosen to obey a barotropic equation of state. Exact solutions of Einstein’s field equations are obtained assuming the volume expansion θ proportional to the eigen values of shear tensor σ _ij . Physical and kinematical properties of the models are discussed considering bulk viscosity to be a power function of matter density.     

Theoretical investigations on the elastic and thermodynamic properties of Ti2AlC0.5N0.5 solid solution

We have performed theoretical studies on the elastic and thermodynamic properties of the solid solution: Ti₂AlC_0.5N_0.5. The lattice parameters, elastic constants, bulk, shear, Young's moduli, Poisson's ratio and Debye temperature were calculated and compared with those of the end members, Ti₂AlC and Ti₂AlN. The temperature dependence of the bulk moduli, thermal expansion coefficient and specific heats of Ti₂AlC_0.5N_0.5 were obtained from the quasi-harmonic Debye model. The calculated elastic and thermodynamic properties were compared with experimental data.     

Structural,elastic and electronic properties of XNCa3 (X = Ge,Sn and Pb) compounds

Using ab initio calculations, we have studied the structural, elastic and electronic properties of XNCa₃, with X=Ge, Sn and Pb. Geometrical optimization of the unit cell are in agreement with the available experimental data. The band structures show that all studied materials are electrical conductors. The analysis of the site and momentum projected densities, charge transfer and total valence charge density shows that the chemical bonding in XNCa₃ compounds is of covalent–ionic nature with the presence of metallic character. The elastic constants and their pressure dependence are calculated using the static finite strain technique. We derived the bulk, shear and Young’s moduli for ideal polycrystalline XNCa₃ aggregates. By analysing the ratio between the bulk and shear moduli, we conclude that XNCa₃ compounds are brittle in nature. We estimated the Debye temperature of XNCa₃ from the average sound velocity.     

First-principles investigation of the equation of state and elastic properties of perovskite-type SrW(O,N)<Subscript>3</Subscript> under hydrostatic pressures up to 139 GPa

Pressure dependence of the structural and elastic properties of perovskite-type cubic SrWO_2.05N_0.95 was studied using firstprinciples density functional theory (DFT) utilizing the plane wave pseudopotential and the exchange-correlation functionals within the generalized gradient approximation. The estimated bulk modulus and its pressure derivative values from the P ? V data fitted to the third-order Birch-Murnaghan equation of state were close to the data obtained from the independent elastic constants. Based on the generalized Born stability criteria, SrWO_2.05N_0.95 is mechanically stable up to 139 GPa. The influence of hydrostatic pressure (0 to 139 GPa) on the bulk modulus, shear modulus, Young’s modulus, Pugh’s modulus ratio, Poisson’s ratio, Vickers hardness, sound velocities, Debye temperature, Debye-Grüneisen parameter, minimum thermal conductivity and elastic anisotropy of SrWO_2.05N_0.95 was particularly studied in detail. It was found that SrWO_2.05N_0.95 is a ductile and hard solid with large bulk, shear and Young’s modulus and displays an extraordinary low thermal conductivity. Since there are not any experimental or theoretical data available for comparison the results of the present study have revealed an important fundamental information about the elastic properties of perovskite-type cubic SrWO_2.05N_0.95 for future experimental studies.     

Nuclear ground-state properties in a relativistic Meson-Field theory

We investigate the ability of a relativistic Mean-Field theory to reproduce nuclear ground state properties by an exhaustive fit to experimental data. We find that the bulk properties of nuclei from¹⁶O to²⁰⁸Pb can be adjusted very well. There remain problems with level density and fluctuations in the charge density similar as in fits using the conventional Skyrme Hartree-Fock model.     

Lattice-Boltzmann Simulations of Particle-Fluid Suspensions

This paper reviews applications of the lattice-Boltzmann method to simulations of particle-fluid suspensions. We first summarize the available simulation methods for colloidal suspensions together with some of the important applications of these methods, and then describe results from lattice-gas and lattice-Boltzmann simulations in more detail. The remainder of the paper is an update of previously published work,^{(69, 70)} taking into account recent research by ourselves and other groups. We describe a lattice-Boltzmann model that can take proper account of density fluctuations in the fluid, which may be important in describing the short-time dynamics of colloidal particles. We then derive macro-dynamical equations for a collision operator with separate shear and bulk viscosities, via the usual multi-time-scale expansion. A careful examination of the second-order equations shows that inclusion of an external force, such as a pressure gradient, requires terms that depend on the eigenvalues of the collision operator. Alternatively, the momentum density must be redefined to include a contribution from the external force. Next, we summarize recent innovations and give a few numerical examples to illustrate critical issues. Finally, we derive the equations for a lattice-Boltzmann model that includes transverse and longitudinal fluctuations in momentum. The model leads to a discrete version of the Green–Kubo relations for the shear and bulk viscosity, which agree with the viscosities obtained from the macro-dynamical analysis. We believe that inclusion of longitudinal fluctuations will improve the equipartition of energy in lattice-Boltzmann simulations of colloidal suspensions.