Ian K. Snook

Solvation forces in simple dense fluids. I

The grand canonical ensemble Monte Carlo method is used to calculate the density profile of a simple dense liquid, under conditions close to the vapor line, between two solid bodies and also the solvation force between the solids due to the simple fluid. The force is large compared with the van der Waals force at moderate surface separations, h, but is an oscillatory function of h. At small values of h the solvation force is strongly repulsive.

On fitting a gold embedded atom method potential using the force matching method

We fit a new gold embedded atom method (EAM) potential using an improved force matching methodology which included fitting to high-temperature solid lattice constants and liquid densities. The new potential shows a good overall improvement in agreement to the experimental lattice constants, elastic constants, stacking fault energy, radial distribution function, and fcc/hcp/bcc lattice energy differences over previous potentials by Foiles, Baskes, and Daw (FBD) [Phys. Rev. B 33, 7983 (1986)] Johnson [Phys. Rev. B 37, 3924 (1988)], and the glue model potential by Ercolessi et al. [Philos. Mag. A 50, 213 (1988)]. Surface energy was improved slightly as compared to potentials by FBD and Johnson but as a result vacancy formation energy is slightly inferior as compared to the same potentials. The results obtained here for gold suggest for other metal species that further overall improvements in potentials may still be possible within the EAM framework with an improved fitting methodology. On the other hand, we also explore the limitations of the EAM framework by attempting a brute force fit to all properties exactly which was found to be unsuccessful. The main conflict in such a brute force fit was between the surface energy and the liquid lattice constant where both could not be fitted identically. By intentionally using a very large number of spline sections for the pair potential, electron-density function, and embedding energy function, we eliminated a lack of functional freedom as a possible cause of this conflict and hence can conclude that it must result from a fundamental limitation in the EAM framework.

Precursor-mediated crystallization process in suspensions of hard spheres.

We report on a large scale computer simulation study of crystal nucleation in hard spheres. Through a combined analysis of real- and reciprocal-space data, a picture of a two-step crystallization process is supported: First, dense, amorphous clusters form which then act as precursors for the nucleation of well-ordered crystallites. This kind of crystallization process has been previously observed in systems that interact via potentials that have an attractive as well as a repulsive part, most prominently in protein solutions. In this context the effect has been attributed to the presence of metastable fluid-fluid demixing. Our simulations, however, show that a purely repulsive system (that has no metastable fluid-fluid coexistence) crystallizes via the same mechanism.

The grand canonical ensemble Monte Carlo method applied to the electrical double layer

The grand canonical ensemble Monte Carlo method is applied to the electrical double layer, using a discrete surface charge distribution and the restricted primitive model for the electrolyte. A systematic examination of the effects of the dimensions of the basic Monte Carlo cell shows that the usual minimum image technique, for determining the potential energy of the system, must be supplemented by long range corrections. Alternatively, very large dimensions or numbers of ions must be employed. The calculated ion distribution functions are compared with the Gouy–Chapman theory for bulk electrolyte concentrations up to 0.2 C/m2. The Gouy–Chapman theory compares very well with the Monte Carlo results for electrolyte concentrations up to 0.1 mole/dm3, but quantitatvive differences appear at 1 mole/dm3. These differences are more pronounced at 2 mole/dm3 for which the Monte Carlo results indicate an inversion of the positive and negative charge distribution functions.

Structure of dense liquids at solid interfaces

Detailed calculations of dense fluids of hard spheres, soft repulsive spheres and Lennard‐Jones molecules between hard, soft repulsive and attractive solid walls reveals a pronounced stratification of the fluid’s density profile. The calculations have been performed using the NVT Monte Carlo method with about 200 fluid molecules. The results indicate that a very significant role is played by the attractive component in the intermolecular potentials, which suggests that the usual hard‐sphere perturbation theories are not applicable in their present form.

Structural Relaxation and Relative Stability of Nanodiamond Morphologies

Presented here are the results of ab initio Density Functional Theory (DFT) relaxations performed on nanocrystalline diamond structures of cubic {100}, octahedral {111} and cuboctahedral morphologies, up to approximately 1 nm in diameter. Results show that in this size range, the crystal morphology plays an important role in the structural stability of the crystals, in the absence of external fields. While the surfaces of the cubic crystals exhibited reconstruction and relaxations comparable to that of bulk diamond, the surfaces of the octahedral and cuboctahedral crystals showed the transition from sp3 to sp2 bonding. Our results demonstrate the inward transition of nanodiamond clusters into carbon onion-like structures, with preferential exfoliation of the (111) surfaces, in agreement with recent experimental observations. The results of this study will provide a better understanding of the effects of nanodiamond morphology on the stability of diamondoid nanostructures and nanodevices.

Hybrid approach for generating realistic amorphous carbon structure using metropolis and reverse Monte Carlo

An improved method for the modelling of carbon structures based on a hybrid reverse Monte Carlo (HRMC) method is presented. This algorithm incorporates an accurate environment dependent interaction potential (EDIP) in conjunction with the commonly used constraints derived from experimental data. In this work, we compare this new method with other modelling results for a small system of 2.9 g/cc amorphous carbon. We find that the new approach greatly improves the structural description, alleviating the common problem in standard reverse Monte Carlo method (RMC) of generating structures with a high proportion of unphysical small rings. The advantage of our method is that larger systems can now be modelled, allowing the incorporation of mesoscopic scale features.

Size dependent phase stability of carbon nanoparticles: Nanodiamond versus fullerenes

Over the past 15 years, a number of studies have reported findings comparing the relative stability of diamond and graphite, at the nanoscale. In light of more recent experimental and theoretical results concerning the transformation of nanodiamonds into carbon-onions, it is considered important to extend this body of work to included fullerenes. Presented here is a study of the phase stability of carbon nanoparticles, with particular attention given to the relative stability of nanodiamonds and fullerenes. The structural energies have been calculated using density functional theory within the generalized gradient approximation using the Vienna ab initio simulation package, and used to determine the standard heat of formation for respective carbon phases as a function of the number of carbon atoms. Our results show that in contrast to previously reported studies, nanodiamond is not necessarily the stable phase a the nanoscale, but instead occupies a “window” of stability between ∼1.9 and ∼5.2 nm.

Application of numerical basis sets to hydrogen bonded systems: A density functional theory study

We have investigated and compared the ability of numerical and Gaussian-type basis sets to accurately describe the geometries and binding energies of a selection of hydrogen bonded systems that are well studied theoretically and experimentally. The numerical basis sets produced accurate results for geometric parameters but tended to overestimate binding energies. However, a comparison of the time taken to optimize phosphinic acid dimer, the largest complex considered in this study, shows that calculations using numerical basis sets offer a definitive advantage where geometry optimization of large systems is required.

Physical adsorption of gases at high pressure: III. Adsorption in slit-like pores

A grand canonical ensemble Monte Carlo study is made of the adsorption of a gas of simple molecules in the space between two infinite, flat, parallel surfaces (slit-like pores) as a function of their separation, h, at various temperatures. Above the critical temperature the overall shape of the adsorption isotherm is independent of h but both the maximum adsorption and the pressure, at which the maximum occurs, decrease as h decreases. Below the critical temperature the adsorption isotherm follows that obtained for an isolated surface up to a pressure p 1 beyond which the adsorption excess increases sharply with pressure to a plateau value. This rapid increase is interpreted in terms of condensation of the gas in the cavity between the surfaces, i.e. capillary condensation. As h is decreased the value of p 1 decreases. When h is comparable with the diameter of the adsorbate molecules the cavity is full at all readily attainable pressures. The results are further interpreted in terms of the singlet distrib...