Equation of state and interaction potential of helium under high temperatures and high densities

Based on the thermodynamics statistic method, the improved variational perturbation theory and the modified quantum mechanics correction model have been used to calculate the equation of state of liquid helium at pressure from 0.7 to 108 GPa. The calculation results are in good agreement with the experimental data. The EXP-6 potential (α = 13.1) can more accurately describe the interaction of helium atoms than other potentials in the scheme. Finally, a comparison is shown between our interatomic potentials and other potentials.     

A new scheme for calculation of effective pair potentials for liquid metals

A new scheme based on the Singwi et al. theory for an interacting electron gas is proposed for calculating pair potentials for liquid metals and applied to liquid sodium and rubidium. The resulting pair potentials are in much better agreement with the pseudopotential calculations than any of the three usual approximations, PY, HNC or BG used in the literature.     

Molecular dynamics study of electron gas models for liquid water

The frozen density electron gas model proposed by Gordon and Kim for rare gas systems has been implemented in a molecular dynamics code. This code has been applied to investigate various options for extending this scheme to inter-molecular interactions in liquid water. We have compared a number of gradient corrections to the Thomas-Fermi kinetic energy. We also explored a more empirical approach based on adaptation of the frozen molecular electron density to the condensed phase environment. Consistent with experience from force field methods, enhancement of the molecular dipole moment proved to be necessary to reproduce the properties of the liquid. The best models we investigated are a gradient corrected expansion of the simple local density Hamiltonian applied in the original Gordon and Kim model. In addition, these models observed a modified molecular electron density carrying the same dipole moment of 2.95 D as has been observed by recent ab initio molecular dynamics studies based on fully self-consistent Kohn-Sham methods. Possible implications of this finding for force field models are discussed.     

Spheroidally deformed sodium clusters in the selfconsistent jellium model

We present the first systematic study of potential energy curves and prolate-oblate shape transitions of sodium clusters with 8 < N < 40 atoms. The Kohn-Sham equations are solved in the local density approximation for the jellium model with spheroidal deformations. The ionic background density is taken to have a diffuse surface of Woods-Saxon type. The quadrupole and hexadecupole moments of the electron and jellium densities are investigated, revealing a strong hexadecupole dependence for selected clusters. Collective dipole resonances are described in the simple surface plasmon model. Shape transitions are found to occur at particle numbers 12–14 (prolate-oblate), 18–20–22 (oblate-spherical-prolate) and 30–32 (prolate-oblate), which are in good agreement with experimental results; triaxiality is predicted for Na-36. Comparing our results with those of molecular dynamics calculations, we confirm the scheme of Kohn-Sham levels and the gross behaviour of potentials and densities.     

MOLECULAR DYNAMICS STUDY OF LIQUID ALUMINIUM BY EMBEDDED-ATOM METHOD

A molecular-dynamics scheme in the embedded-atom method is shown to be the efficient and accurate in studying liquid aluminium system. The ability of the method in studying liquid system is demonstrated by calculating the intent heat, self-diffusion coefficient, pair distribution function of aluminium, and so on. All of the results agree well with experimental results. The background electron density is also calculated using this method, and the result shows that a discontinuous change occurs at melting point for the electron density in the system.     

Problems in the use of statistical average atom potentials for estimating average degree of ionization

Consequences of a simple integral definition of electron charge bound to an ion are examined for Thomas-Fermi (TF) and Debye-Huckel-Thomas-Fermi (DHTF) average atom statistical potentials used to describe high temperature high density plasmas. A self-consistent scheme for calculating average degree of ionization within the DHTF approach is described. With the simple integral definition of bound charge the DHTF model, unlike the TF model, exhibits the anomalous behavior that degree of ionization can decrease as temperature increases. It is shown that this results from inclusion in the integration of electron charge density too extended and too near continuum energies to be physically considered as bound.     

Universality in the effective pair interaction ofd-shell metals — Compressibility and vacancy formation energy of 3d-liquid metals

Like liquid alkali metalsd-shell liquid metals show scaling behaviour in structure and interaction potentials. A realistic interaction potential model, properly parametrized can reasonably describe the universality in the isothermal compressibility and vacancy formation energy of 3d-liquid metals in electron ion plasma model.     

Universal Scaling Law for Atomic Diffusion and Viscosity in Liquid Metals

The recently proposed scaling law relating the diffusion coefficient and the excess entropy of liquid [Samanta A et al. 2004 Phys. Reu. Lett. 92 145901; Dzugutov M 1996 Nature 381 137], and a quasi-universal relationship between the transport coefficients and excess entropy of dense fluids [Rosenfeld Y 1977 Phys. Rev. A 15 2545],are tested for diverse liquid metals using molecular dynamics simulations. Interatomic potentials derived from the glue potential and second-moment approximation of tight-binding scheme are used to study liquid metals.Our simulation results give sound support to the above-mentioned universal scaling laws. Following Dzugutov,we have also reached a new universal scaling relationship between the viscosity coefficient and excess entropy.The simulation results suggest that the reduced transport coefficients can be expressed approximately in terms of the corresponding packing density.     

The Effect of Rotating Cylindrical Langmuir Probes in Magnetron Plasmas

Using a cylindrical Langmuir probe, the plasma properties (ion density, electron temperature, floating and plasma potentials) in a magnetron sputter source have been investigated, along one particular line‐of‐sight, but for different probe‐orientations with respect to the B‐field. The plasma in the region hosen for observation is haracterised by electrons, which are magnetised (Larmor radius r_le < both the electron mean‐free‐path λ_e, and plasma extension L) and ions, which are not (their Larmor radius r_I > L, λ_I). Through the development of a simple expression for the electron saturation current at different probe angles relative to the local B‐field, it is possible to correct for the diminished electron currents due to their restricted transport across the field. The results indicate that the measured ion density, electron temperature, floating and plasma potentials are unaffected by the probe orientation, however the electron saturation current is attenuated when the probe is aligned along the B‐field. A simple model for the collection of electrons indicates that classical electron diffusion may not operate in the magnetron, with cross‐field electron transport dominated by anomalous, possibly Bohm, diffusion.     

Electronic theory for the metal-non-metal transition in simple metal alloys

The metal-non-metal transition in liquid alloys such as Cs_1–x Au _x and Li_1–x Pb _x is explained as resulting from concentration dependent electron charge transfer causing short range atomic order and a change from metallic to ionic bonds. Numerical results for the electronic density of states, the electron charge transfer, the free energy of mixing and the volume change are given.     

Nuclear quantum effects in liquid water from path-integral simulations using an ab initio force-matching approach

We have applied path integral simulations, in combination with new ab initio based water potentials, to investigate nuclear quantum effects in liquid water. Because direct ab initio path integral simulations are computationally expensive, a flexible water model is parameterised by force-matching to density functional theory-based molecular dynamics simulations. Static and dynamic properties of liquid water at ambient conditions are presented and the role of nuclear quantum effects, exchange-correlation functionals and dispersion corrections are discussed in regards to reproducing the experimental properties of liquid water.     

An efficientmethod to determine chemical potential of mixtures in the isothermal and isobaric bulk phase with kineticMonte Carlo simulation

A kinetic Monte Carlo (kMC) scheme in the NPT ensemble (constant number of molecules, pressure and temperature) has been developed to determine accurate chemical potentials for all components in a homogeneous mixture. The simulation requires two moves: (1) a displacement move and (2) a volume change move. In the former, the mobility rate of a selected molecule is determined by its interaction with all the other molecules in the system and is moved to a random position within the simulation box, according to the Rosenbluth algorithm, without any rejections (entropic sampling). The volume change move is decided by a comparison between either the instant pressure or the partial average pressure (with long-range correction) and the specified pressure and is carried out much less frequently than the displacement move. We applied this NPT scheme to a number of mixtures in both the gaseous and liquid phases, and show that the derived chemical potentials are accurate and reproducible. The method is recommended for obtaining chemical potentials in mixtures that are required as input in a grand canonical ensemble simulation.     

The O-H distance in ice

The thermodynamic properties of normal and para-hydrogen are computed from multiple time-step path integral hybrid Monte Carlo (PIHMC) simulations. Four different isotropic pair potentials are evaluated by comparing simulation results with experimental data. The Silvera–Goldman potential is found to be the most accurate of the potentials tested for computing the density and internal energy of fluid hydrogen. Using the Silvera–Goldman potential, simulation and experimental data are compared on isobars ranging from 0.1 to 100 MPa and for temperatures from 18 to 300 K. The Gibbs free energy is calculated from the PIHMC simulations by an adaptation of Widom's particle insertion technique to a path integral fluid. A new method is developed for computing phase equilibria for quantum fluids directly by combining PIHMC with the Gibbs ensemble technique. This Gibbs–PIHMC method is used to calculate the vapour–liquid phase diagram of hydrogen from simulations. Agreement with experimental data is good.     

Characteristic Topological and Energy Features of the Positioning and Transport Potentials in Nanosystems

Topological and energy specifics of interparticle potentials in the computer modeling of block confinement, positioning, and transport processes inside low-dimensional nanosystems are considered. It is shown that inter- and intrablock bonds in nanostructures can be determined in the context of the theory of quantum topology of electron density and the method of electron density functional. The results of calculations of interatomic transport potentials inside the carbon, silicon, and aluminum nanoblocks are presented. It is shown that the C–C potentials in nanostructures have an activation barrier. As a consequence, the transportable C atoms are unique in forming low-dimensional nanostructures. The equilibrium parameters in Al and Si nanosystems differ only insignificantly from the corresponding equilibrium crystal parameters. The results of computer modeling of the effect of strong electron correlation on the metal-insulator conversion in the transport system electron-nanotube are analyzed.     

Many-particle-effects in the theory of the extended X-ray absorption fine structure

The Lee-Beni-procedure for the calculation of the extended X-ray absorption fine structure (EXAFS) is extended so as to include the effects of the electronic charge density outside the localized muffin-tin potentials. In our scheme EXAFS is caused by back-scattering of an elementary excitation of a homogeneous electron gas by localized energy dependent many-particle muffin-tin potentials. The difference between the two schemes is negligible at large k's, as expected from physical grounds. However, at small and intermediate k-values the difference is quite large. The effect of the outer electrons as compared to the Lee-Beni-model is twofold. First, they renormalize the scattered electron in the usual way. Second, they are missing within the scattering muffin-tins. Hence, we avoid to count some of the electrons twice. Results are presented for Cu as an example.     

Grand Equilibrium: vapour-liquid equilibria by a new molecular simulation method

A new molecular simulation method for the calculation of vapour-liquid equilibria of mixtures is presented. In this method, the independent thermodynamic variables are temperature and liquid composition. In the first step, one isobaric isothermal simulation for the liquid phase is performed, in which the chemical potentials of all components and their derivatives with respect to the pressure, i.e. the partial molar volumes, are calculated. From these results, first-order Taylor series expansions for the chemical potentials as functions of the pressure μ _i (p) at constant liquid composition are determined. This information is needed, as the specified pressure in the liquid will generally not be equal to the equilibrium pressure, which has to be found in the course of a vapour simulation. In the second step, one pseudo grand canonical simulation for the vapour phase is performed, where the chemical potentials are set according to the instantaneous pressure p ^v using the previously determined function μ _i (p ^v). In this way, results for the vapour pressure and vapour composition are achieved which are consistent for the given temperature and liquid composition. The new method is applied to the pure Lennard-Jones fluid, a binary and a ternary mixture of Lennard-Jones spheres, and shows very good agreement with corresponding data from the literature.     

The Fisher-Widom line for systems with low melting temperature

We present Monte Carlo results for the pair distribution function of three simple fluid models, with pairwise interactions, which have low triple point temperatures and mimic some aspects of Na and Hg liquid metals. The results are then used to get the direct correlation function, by numerical solution of the Ornstein-Zernike equation, and to characterize the decay modes of any density distribution towards the bulk fluid. The Fisher-Widom line is obtained from the crossing of the two lowest inverse decay lengths, associated to monotonic and to oscillatory decay modes. For the pair potential models with a soft repulsive core, the Fisher-Widom line appears well below the critical temperature and has positive slope of the temperature with respect to the density, contrary to previous results for the Lennard-Jones and square-well potentials which had located that line quite close to T _c and with negative slope.     

New correlation potential for the local-spin-density functional formalism I. Total energies and ionization potentials in free atoms

Based on the recently proposed Vosko-Wilk-Nusair interpolation formulae for the correlation energy density of the spin-polarized homogeneous electron liquid, a new parametrized form for the correlation potential for the self-consistent local-spin-density calculations of atoms, molecules and solids is proposed. The total energies and first ionization potentials for a few light atoms are calculated. The influence of the improved spin-polarization dependence on the calculated quantities is investigated, too. The calculated ionization potentials are in good agreement with observed values and are better (in average by 0.3 eV) than the values predicted by the hitherto most commonly used correlation potentials within the local spin density approximation.