Electron emission from diamondoids: a diffusion quantum Monte Carlo study

We present density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations designed to resolve experimental and theoretical controversies over the optical properties of H-terminated C nanoparticles (diamondoids). The QMC results follow the trends of well-converged plane-wave DFT calculations for the size dependence of the optical gap, but they predict gaps that are 1-2 eV higher. They confirm that quantum confinement effects disappear in diamondoids larger than 1 nm, which have gaps below that of bulk diamond. Our QMC calculations predict a small exciton binding energy and a negative electron affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized nature of the lowest unoccupied molecular orbital. The NEA suggests a range of possible applications of diamondoids as low-voltage electron emitters.     

Surface tension of an electrolyte-air interface: a Monte Carlo study

We present a new method for calculating the surface tension of an electrolyte-air interface using Monte Carlo (MC) simulations with an implicit solvent in a spherical drop geometry. The boundary conditions for the electric field at the interface are accounted for using image and counter-image charges. The density profiles obtained from the simulations are used to calculate the excess surface tension of the electrolyte-air interface using the Gibbs adsorption isotherm equation. The results are found to be in good agreement with experiments and the earlier theoretical calculations.     

A diffusion Monte Carlo study of small para-hydrogen clusters

An improved Monte Carlo diffusion model is used to calculate the ground state energies and chemical potentials of parahydrogen clusters of three to forty molecules, using two different p-H₂-p-H₂ interactions. The improvement is due to three-body correlations in the importance sampling, to the time step adjustment and to a better estimation of statistical errors. In contrast to path-integral Monte Carlo results, this method predicts no magic clusters other than that with thirteen molecules.      

Sensitivity analysis of a Monte Carlo atmospheric diffusion model

Summary A Monte Carlo model simulating atmospheric transport and diffusion in the PBL is presented. In a previous study, it was shown that it works well in homogeneous turbulence and fits well the Willis and Deardorff water tank simulations in convective nonhomogeneous conditions. In the present paper a sensitivity analysis aimed at estimating the importance and effectiveness of the model parameters is performed and discussed. Our model makes use of Hanna's scheme for the vertical structure of the turbulent parameters, and of an empirical parametrization of updrafts and downdrafts in convective unstable conditions. Emitting the particles either from a point source or uniformly distributed along the vertical we found that the model avoids the particle accumulation at the top and bottom of the PBL and recovers the Eulerian turbulent statistics. This demonstrates that our numerical scheme is consistent with the physical constraints,i.e. mass and energy are preserved and an initial uniform distribution remains so. Finally it is shown that considering the contribution of the cross-correlation term does not improve significantly the model performances.

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Riassunto Si presenta un modello Monte Carlo atto a simulare il trasporto e la diffusione nello strato limite atmosferico che era stato validato in uno studio precedente ove si era trovato che esso lavora bene in condizioni di turbolenza omogenea e riproduce bene i risultati delle misure in vasca idraulica di Willis e Deardorff relativi a condizioni di turbolenza non omogenea. Nel presente lavoro è stata effettuata un'analisi di sensitività del modello volta alla valutazione dell'importanza ed efficacia dei parametri del modello stesso. Esso utilizza lo schema di Hanna per la descrizione dell'andamento verticale dei parametri turbolenti in atmosfera ed una parametrizzazione empirica delle celle convettive in condizioni instabili. Emettendo le particelle sia da una sorgente puntiforme, sia distribuite uniformemente lungo la verticale si è trovato che il modello evita l'accumulo di particelle ai limiti superiore ed inferiore del PBL e conserva la statistica euleriana delle grandezze turbolente. Ciò dimostra che il nostro schema numerico è fisicamente consistente dato che soddisfa i seguenti vincoli fisici: la massa e l'energia sono conservate ed una distribuzione iniziale uniforme rimane tale. Infine, si è mostrato che il considerare il contributo del termine di cross-correlazione non migliora in modo significativo le prestazioni del modello.

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Резюме Предлагается модель Моите Карло для моделирования процессов диффузии и переноса в атмосфере. Ранее было показано, что эта модель хорошо работает в условиях однородной турбулентности и хорошо воспроизводит результаты Виллиса и Дирдорфа измерений в водоеме при условиях неоднородной турбулентности. В данной статье проводится и обсуждается анализ чувствительности с целью определения важности и эффективности параметров модели. В предложенной модели используется схема Хана для вертикальной структуры параметров турбулентности атмосферы и эшпирическая параметризация для конвективных ячеек в нестационарных условиях. Испуская частиы либо из точечного источника, либо из однородно распределенных источников вдоль вертикали, мы находим, что модель не дает накапливания частиц на верхней и нижней границах PBL и сохраняет Эйлерову статистику турбулентности. Мы показываем, что наша численная схема согласуется с физическими ограничениями: масса и энергия сохраняются и начальное однородное распределение остается тем же. В заключение покаывается, что рассмотрение вклада перекрестного члена значительно не улучшает характеристики модели.

     

First-passage Monte Carlo algorithm: diffusion without all the hops

We present a novel Monte Carlo algorithm for N diffusing finite particles that react on collisions. Using the theory of first-passage processes and time dependent Green's functions, we break the difficult N-body problem into independent single- and two-body propagations circumventing numerous diffusion hops used in standard Monte Carlo simulations. The new algorithm is exact, extremely efficient, and applicable to many important physical situations in arbitrary integer dimensions.     

Fixed-phase diffusion Monte Carlo study of the quantum-Hall effect on the Haldane sphere

A generalized diffusion Monte Carlo method for solving the many-body Schrödinger equation on curved manifolds is introduced and used to perform a ‘fixed-phase’ simulation of the integer and fractional quantum-Hall effect on the Haldane sphere. The effect of Landau level mixing on the and energy gaps and the relative stability of spin-polarized and spin-reversed quasielectron excitations are studied using the new method.     

Quantum ice: a quantum Monte Carlo study

Ice states, in which frustrated interactions lead to a macroscopic ground-state degeneracy, occur in water ice, in problems of frustrated charge order on the pyrochlore lattice, and in the family of rare-earth magnets collectively known as spin ice. Of particular interest at the moment are "quantum spin-ice" materials, where large quantum fluctuations may permit tunnelling between a macroscopic number of different classical ground states. Here we use zero-temperature quantum Monte Carlo simulations to show how such tunnelling can lift the degeneracy of a spin or charge ice, stabilizing a unique "quantum-ice" ground state-a quantum liquid with excitations described by the Maxwell action of (3+1)-dimensional quantum electrodynamics. We further identify a competing ordered squiggle state, and show how both squiggle and quantum-ice states might be distinguished in neutron scattering experiments on a spin-ice material.     

Alternate Monte Carlo algorithm applied to a Lennard-Jones solid

Monte Carlo calculations of the thennodynamic properties of a classical nearest neighbor Lennard-Jones solid have been carried out using an alternate to the usual Metropolis algorithm. The new algorithm is particularly convenient for the study of solids. Unbiased estimators are used to obtain values for the properties and their standard deviations. Our results are combined with recent results of Cowley to give a set of best estimates.     

Monte Carlo PDF modelling of a turbulent natural-gas diffusion flame

A piloted turbulent natural-gas diffusion flame is investigated numerically using a 2D elliptic Monte Carlo algorithm to solve for the joint probability density function (PDF) of velocity and composition. Results from simulations are compared to detailed experimental data: measurements of temperature statistics, data on mean velocity and turbulence characteristics and data on OH. Conserved-scalar/constrained-equilibrium chemistry calculations were performed using three different models for scalar micro-mixing: the interaction by exchange with the mean (IEM) model, a coalescence/dispersion (C/D) model and a mapping closure model. All three models yield good agreement with the experimental data for the mean temperature. Temperature standard deviation and PDF shapes are generally predicted well by the C/D and mapping closure models, whereas the IEM model gives qualitatively incorrect results in parts of the domain. It is concluded that the choice of micro-mixing model can have a strong influence on the quality of the predictions. The same flame was also simulated using reduced chemical kinetics obtained from the intrinsic low-dimensional manifold (ILDM) approach. Comparison with the constrained-equilibrium results shows that the shape of the OH concentration profiles is recovered better in the ILDM simulation, and that the ILDM reduced chemical kinetics can correctly predict super-equilibrium OH.     

Hellman-Feynman operator sampling in diffusion Monte Carlo calculations

Diffusion Monte Carlo (DMC) calculations typically yield highly accurate results in solid-state and quantum-chemical calculations. However, operators that do not commute with the Hamiltonian are at best sampled correctly up to second order in the error of the underlying trial wave function once simple corrections have been applied. This error is of the same order as that for the energy in variational calculations. Operators that suffer from these problems include potential energies and the density. This Letter presents a new method, based on the Hellman-Feynman theorem, for the correct DMC sampling of all operators diagonal in real space. Our method is easy to implement in any standard DMC code.     

Monte Carlo simulation to study surface diffusion and reaction processes on a fractal catalyst

The surface of a catalyst is a fractal, and its fractal dimension can strongly influence the performance of a catalytic reaction. By means of the Monte Carlo method, the decompositions of N₂O on a square lattice and a fractal surface are investigated in this paper. From the simulation it is found that the fractal surface can change both the reaction rate constant K and the reaction order α in the N₂O decomposition, and all of these changes are caused by the abnormal diffusion of the surface species on the fractal surface.     

High-temperature superconductivity in the apex-oxygen model: a quantum Monte Carlo study

The Projector Quantum Monte Carlo Method is applied to examine the existence of superconductivity in the Apex-Oxygen-Model which describes the physics of the correlated carriers in the CuO₂-planes coupled to the anharmonic vibrations of the apex oxygen in the hightemperature superconductors. The simulations show clear evidence for extendeds-wave superconductivity in the 100 K temperature range for realistic choice of coupling parameters.     

蒙特卡洛方法计算材料表面激发参数

电子非弹性散射平均自由程（IMFP）是用表面电子能谱进行表面化学定量分析时极为重要的一个参数,它可以用测量的弹性峰电子能谱分析以及蒙特卡洛模拟来确定.为了更加精确地确定电子非弹性散射平均自由程,必须对弹性峰电子能谱中的表面激发效应进行修正,通常使用介电响应理论方法计算得到的表面激发参数.然而,通过理论计算得到的表面激发参数不能包含电子在材料内部输运过程中弹性散射的影响,进而影响所测的电子非弹性散射平均自由程的准确度.在这个工作中,我们采用蒙特卡洛方法来确定包含弹性散射效应时的表面激发参数.所得到的表面激发参数在不同能量、角度情况下,尤其是在弹性散射效应显著的60°以上的大角度入射、出射情况下,都与实验测量值符合得非常好.基于这些新确定的表面激发参数,可以在弹性峰电子能谱测量中获得更为准确的电子非弹性散射平均自由程数据.     

Impurity transport in fractal media in the presence of a degrading diffusion barrier

We have analyzed the transport regimes and the asymptotic forms of the impurity concentration in a randomly inhomogeneous fractal medium in the case when an impurity source is surrounded by a weakly permeable degrading barrier. The systematization of transport regimes depends on the relation between the time t ₀ of emergence of impurity from the barrier and time t _* corresponding to the beginning of degradation. For t ₀ < t _*, degradation processes are immaterial. In the opposite situation, when t ₀ > t _*, the results on time intervals t < t _* can be formally reduced to the problem with a stationary barrier. The characteristics of regimes with t _* < t < t ₀ depend on the scenario of barrier degradation. For an exponentially fast scenario, the interval t _* < t < t ₀ is very narrow, and the transport regime occurring over time intervals t < t _* passes almost jumpwise to the regime of the problem without a barrier. In the slow power-law scenario, the transport over long time interval t _* < t < t ₀ occurs in a new regime, which is faster as compared to the problem with a stationary barrier, but slower than in the problem without a barrier. The asymptotic form of the concentration at large distances from the source over time intervals t < t ₀ has two steps, while for t > t ₀, it has only one step. The more remote step for t < t ₀ and the single step for t > t ₀ coincide with the asymptotic form in the problem without a barrier.