Showing papers by "Lev D. Gelb published in 2012"
TL;DR: These simulations show that a variety of interesting clathrate structures and phase behaviors may be accessed in suitably designed microporous materials, with potentially useful applications in gas storage or separations.
Abstract: Monte Carlo simulations are used to study the structure, stability, and dissociation mechanisms of methane hydrate crystals inside carbon-like slit-shaped pores. The simulation conditions used mimic experimental studies of the dissociation of methane and propane hydrates in mesoporous silica gels (Handa, Y. P.; Stupin, D. J. Phys. Chem. 1992, 96, 8599). Simulations are performed under conditions of fixed methane pressure and fixed water loading, with the temperature increased in steps, with long equilibrations at each temperature. The initial structures of the confined hydrates are taken to be bulk-like, and pore widths chosen to accommodate integer or half-integer numbers of hydrate unit cells. Density profiles and orientational order parameter profiles are obtained and used to understand the structural changes associated with hydrate dissociation. Three different common water models, SPC/E, TIP4P, and TIP4P/2005, are used and the results compared. For water modeled using either the TIP4P or TIP4P/2005 p...
45 citations
16 Jul 2012
TL;DR: The application of atomistic Monte Carlo simulation based on electronic structure calculations to elemental systems such as metals and alloys is discussed and an approximate "pre-sampling" potential is used to generate large moves with a high probability of acceptance.
Abstract: We discuss the application of atomistic Monte Carlo simulation based on electronic structure calculations to elemental systems such as metals and alloys. As in prior work in this area, an approximate "pre-sampling" potential is used to generate large moves with a high probability of acceptance. Even with such a scheme, however, such simulations are extremely expensive and may benefit from algorithmic developments that improve acceptance rates and/or enable additional parallelization.Here we consider several such developments. The first of these is a three-level hybrid algorithm in which two pre-sampling potentials are used. The lowest level is an empirical potential, and the "middle" level uses a low-quality density functional theory. The efficiency of the multistage algorithm is analyzed and an example application is given.Two other schemes for reducing overall run-time are also considered. In the first, the Multiple-try Monte Carlo algorithm, a series of moves are attempted in parallel, with the choice of the next state in the chain made by using all the information gathered. This is found to be a poor choice for simulations of this type. In the second scheme, "tree sampling," multiple trial moves are made in parallel such that if the first is rejected, the second is ready and can be considered immediately. Performance of this scheme is shown to be quite effective under certain reasonable run parameters.
1 citations