Quantum Monte Carlo simulation of overpressurized liquid 4He

A diffusion Monte Carlo simulation of superfluid 4He at zero temperature and pressures up to 275 bar is presented. Increasing the pressure beyond freezing (approximately 25 bar), the liquid enters the overpressurized phase in a metastable state. In this regime, we report results of the equation of state and the pressure dependence of the static structure factor, the condensate fraction, and the excited-state energy corresponding to the roton. Along this large pressure range, both the condensate fraction and the roton energy decrease but do not become zero. The roton energies obtained are compared with recent experimental data in the overpressurized regime.     

A Monte Carlo simulation study of associated liquid crystals

We have performed a Monte Carlo simulation study of a system of ellipsoidal particles with donor—acceptor sites modelling complementary hydrogen-bonding groups in real molecules. We have considered elongated Gay—Berne particles with terminal interaction sites allowing particles to associate and form dimers. The changes in the phase transitions and in the molecular organization and the interplay between orientational ordering and dimer formation are discussed. Particle flip and dimer moves have been used to increase the convergency rate of the Monte Carlo (MC) Markov chain.     

Monte Carlo simulation of mictomagnets

Monte Carlo simulations of a binary alloy with impurity concentrations between 20 and 45 at.% have been carried out. The proportion of large clusters relative to that of small clusters increases with the number of MC diffusion steps as well as impurity concentration. Magnetic susceptibility peaks become more prominent and occur at higher temperatures with increasing impurity concentration. The different peaks in the susceptibility and specific heat curves seem to correspond to different sized clusters. A freezing model would explain the observed behaviour with the large clusters freezing first and the small clusters contributing to susceptibility (specific heat) peaks at lower temperatures.Contribution No. 153 from the Solid State and Structural Chemistry Unit     

Efficient kinetic Monte Carlo simulation

This paper concerns kinetic Monte Carlo (KMC) algorithms that have a single-event execution time independent of the system size. Two methods are presented—one that combines the use of inverted-list data structures with rejection Monte Carlo and a second that combines inverted lists with the Marsaglia–Norman–Cannon algorithm. The resulting algorithms apply to models with rates that are determined by the local environment but are otherwise arbitrary, time-dependent and spatially heterogeneous. While especially useful for crystal growth simulation, the algorithms are presented from the point of view that KMC is the numerical task of simulating a single realization of a Markov process, allowing application to a broad range of areas where heterogeneous random walks are the dominate simulation cost.     

Monte Carlo simulation of nanometric cutting

Nanometric cutting of single-crystal materials at conventional cutting speeds (5?m?s^?1) is simulated for the first time using a new Monte Carlo method that is applicable to systems that are neither canonical nor microcanonical. This is accomplished by defining a local temperature in the cutting zone using the thermal analysis developed by Komanduri and Hou for conventional machining. Extension of this method to the nanometric regime permits an accurate estimate of the local temperature in cutting. This temperature is then employed in the Boltzmann probability distribution function that is used to determine the acceptance–rejection of Monte Carlo moves in the simulation. Since cutting speed is closely related to cutting temperature, the cutting speed enters the calculation via the thermal analysis equations. The method is applied to nanometric cutting of single-crystal aluminium with the crystal oriented in the (001) plane and cut in the [100] direction. Three positive rake cutting tools, namely 10°, 30° and 45°, are employed to investigate the effect of the rake angle on the forces, the specific energy and the nature of the chip formation. The method is evaluated by direct comparison with corresponding molecular dynamics simulations conducted under the same conditions.     

Monte Carlo simulation of exposure factor

In this paper, we simulate the exposure factor by a simple model of a free-air ionization chamber with the Monte Carlo programme Geant4. Special emphasis is placed on the discussion of the exposure factor related to parameters of the chamber model. The reason for the variation in exposure factor with incident ray energy is also analysed in terms of reaction cross section for different types of reactions. The obtained results indicate that our simulation is accurate in the calculation of the exposure factor and can serve as a reference in designing air ionization chambers.     

Monte Carlo studies of liquid water

Computer simulation studies are reported for the Rowlinson and Ben-Naim and Stillinger models of water-water interactions. Particular attention is given to the effects of altering the size of the system and to accounting for some long-range interactions by including the Onsager reaction field. It is shown that both models give a good qualitative account of the structure of liquid water but that neither is able to describe the high dielectric constant. A particularly sensitive property, the dipole-dipole correlation function, demonstrates the problems encountered in truncating the water interactions. Good agreement between the Rowlinson potential and a modified Hartree-Fock calculation suggests that the Rowlinson model is more accurate than the Ben-Naim and Stillinger form.     

A Monte Carlo simulation of coagulation

A Monte Carlo simulation technique is described for the study of the coagulation of suspended particles. The method is computationally efficient since the particle trajectories are not used to determine coagulations. Instead, pairs of particles are assigned probabilities to coagulate and the evolution is computed as a stochastic Markov game. We also describe a simple analytic method to obtain the stationary distribution of sizes for the various mechanisms of relative particle motion. It is demonstrated that the simulation yields the correct stationary size distribution independent of initial condition.     

Monte Carlo simulation of aorta autofluorescence

Results of numerical simulation of autofluorescence of the aorta by the method of Monte Carlo are reported. Two states of the aorta, normal and with atherosclerotic lesions, are studied. A model of the studied tissue is developed on the basis of information about optical, morphological, and physico-chemical properties. It is shown that the data obtained by numerical Monte Carlo simulation are in good agreement with experimental results indicating adequacy of the developed model of the aorta autofluorescence.     

Multilayer adsorption by Monte Carlo simulation

Adsorption phenomena are characterized by models that include free parameters trying to reproduce experimental results. In order to understand the relationship between the model parameters and the material properties, the adsorption of small molecules on a crystalline plane surface has been simulated using the bond fluctuation model. A direct comparison between the Guggenheim–Anderson–de Boer (GAB) model for multilayer adsorption and computer simulations allowed us to establish correlations between the adsorption model parameters and the simulated interaction potentials.     

Monte Carlo simulation of the enantioseparation process

By means of Monte Carlo simulation, a study of enantioseparation by capillary electrophoresis has been carried out. A simplified system consisting of two enantiomers S (R) and a selector chiral C, which reacts with the enantiomers to form complexes RC (SC), has been considered. The dependence of Δμ$\Delta \mu$ (enantioseparation) with the concentration of chiral selector and with temperature have been analyzed by simulation. The effect of the binding constant and the charge of the complexes are also analyzed. The results are qualitatively satisfactory, despite the simplicity of the model.     

Monte Carlo simulation of photoelectron angular distribution

A practical simulation method has been performed for studies of the influence of surface excitations on the angular distributions of photoelectron peak intensities. The surface effects have been incorporated into simulations by using the surface excitation parameters (SEPs) which have been calculated with the extended Drude dielectric function. Also, elastic scattering cross sections are calculated using the finite difference method for a Hartree-Fock-Wigner-Seitz potential in the Dirac equation to take into account the solid-state effect. Results of Monte Carlo simulations reveal that surface effects lead to a reduction of the intensities at small detection angles and a sharp decrease at large angles since the surface excitation is most probable for glancing electrons. The calculated results taking into account surface effects are in better agreement with the experimental data.     

Monte Carlo simulation of spin-aligned atomic hydrogen

The density dependent ground-state properties of spin-aligned atomic hydrogen are studied using the Monte Carlo technique for 32 and 128 atoms in a cube with periodic boundary conditions. The one-particle density matrix, the two-body correlation function, the structure and pairing function have been evaluated and are compared to other recent work. The total number of particles in the condensate is largest at a density 6·10^–3 Å^–3 and amounts to ⁰=2.03·10^–3Å^–3. In addition, the elementary excitation spectrum is discussed in the framework of the Brueckner-Sawada theory. The correct initial slope of the spectrum is obtained from variational results on the structure function. From these results one may tentatively conclude that the roton like part of the spectrum disappears at densities less than 10^–2 Å^–3.