Simulating fluid-solid equilibrium with the Gibbs ensemble

The Gibbs ensemble is employed to simulate fluid-solid equilibrium for a shifted-force Lennard-Jones system. This is achieved by generating an accurate canonical Helmholtz free-energy model of the (defect-free) solid phase. This free-energy model is easily generated, with accuracy limited only by finite-size effects, by a single isothermal-isobaric simulation at a pressure not too far from coexistence for which the chemical potential is known. We choose to illustrate this method at the known triple-point because the chemical potential is easily calculated from the coexisting gas. Alternatively, our methods can be used to locate fluid-solid coexistence and the triple-point of pure systems if the chemical potential of the solid phase can be efficiently calculated at a pressure not too far from the actual coexistence pressure. Efficient calculation of the chemical potential of solids would also enable the Gibbs ensemble simulation of bulk solid-solid equilibrium and the grand-canonical ensemble simulation of bulk solids

The density and drag of the accretion wake of a massive body moving through a uniform stellar distribution

We calculate the change in density within a uniform distribution of field stars (point masses) caused by a single massive body passing through with a constant velocity. Starting with the simplest case in which the field stars are initially stationary this leads to an infinite density wake behind the body. Introducing a small thermalisation within the field stars removes this infinity whilst leading to similar results off the path of the massive body. Results are in good agreement with those previously derived. An approximation can be made for the density in the thermalised case and this can be used to deduce the force exerted on the massive body due to the drag caused by the accretion wake

Implementing Lanier's patents for stable, safe and economical ultra-short wing vacu- and para-planes

Backyard Technology are interested in aspects of aircraft design described by Edward H Lanier in a series of six patents obtained from 1930 to 1933. Lanier's overall aim was to provide an exceptionally stable aeroplane that would both fly normally and recover from undesirable attitudes without pilot aid. Backyard Technology were specifically interested in Lanier's idea of creating a vacuum cavity in the wing by replacing a section of the upper skin of the wing with a series of angled slats, believing that this wing design would give superior lift and stability compared to typical wing designs

Survey of classical density functionals for modelling hydrogen physisorption at 77 K

This work surveys techniques based on classical density functionals for modeling the quantum dispersion of physisorbed hydrogen at 77 K. Two such techniques are examined in detail. The first is based on the "open ring approximation" (ORA) of Broukhno et al., and it is compared with a technique based on the semiclassical approximation of Feynman and Hibbs (FH). For both techniques, a standard classical density functional is used to model hydrogen molecule-hydrogen molecule (i.e., excess) interactions. The three-dimensional (3D) quantum harmonic oscillator (QHO) system and a model of molecular hydrogen adsorption into a graphitic slit pore at 77 K are used as benchmarks. Density functional results are compared with path-integral Monte Carlo simulations and with exact solutions for the 3D QHO system. It is found that neither of the density functional treatments are entirely satisfactory. However, for hydrogen physisorption studies at 77 K the ORA based technique is generally superior to the FH based technique due to a fortunate cancellation of errors in the density functionals used. But, if more accurate excess functionals are used, the FH technique would be superior

Gas adsorption in active carbons and the slit-pore model 1 : pure gas adsorption

We describe procedures based on the polydisperse independent ideal slit-pore model, Monte Carlo simulation and density functional theory (a 'slab-DFT') for predicting gas adsorption and adsorption heats in active carbons.A novel feature of this work is the calibration of gas-surface interactions to a high surface area carbon, rather than to a low surface area carbon as in all previous work. Our models are used to predict the adsorption of carbon dioxide, methane, nitrogen, and hydrogen up to 50 bar in several active carbons at a range of near-ambient temperatures based on an analysis of a single 293 K carbon dioxide adsorption isotherm. The results demonstrate that these models are useful for relatively simple gases at near-critical or supercritical temperatures

Gas adsorption in active carbons and the slit-pore model 2 : mixture adsorption prediction with DFT and IAST

We use a fast density functional theory (a 'slab-DFT') and the polydisperse independent ideal slit-pore model to predict gas mixture adsorption in active carbons. The DFT is parametrized by fitting to pure gas isotherms generated by Monte Carlo simulation of adsorption in model graphitic slit-pores. Accurate gas molecular models are used in our Monte Carlo simulations with gas-surface interactions calibrated to a high surface area carbon, rather than a low surface area carbon as in all previous work of this type, as described in part 1 of this work (Sweatman, M. B.; Quirke, N. J. Phys. Chem. B 2005, 109, 10381). We predict the adsorption of binary mixtures of carbon dioxide, methane, and nitrogen on two active carbons up to about 30 bar at near-ambient temperatures. We compare two sets of results; one set obtained using only the pure carbon dioxide adsorption isotherm as input to our pore characterization process, and the other obtained using both pure gas isotherms as input. We also compare these results with ideal adsorbed solution theory (IAST). We find that our methods are at least as accurate as IAST for these relatively simple gas mixtures and have the advantage of much greater versatility. We expect similar results for other active carbons and further performance gains for less ideal mixtures

The self-referential method for linear rigid bodies : application to hard and Lennard-Jones dumbbells

The self-referential (SR) method incorporating thermodynamic integration (TI) [Sweatman et al., J. Chem. Phys. 128, 064102 (2008)] is extended to treat systems of rigid linear bodies. The method is then applied to obtain the canonical ensemble Helmholtz free energy of the alpha-N2 and plastic face centered cubic phases of systems of hard and Lennard-Jones dumbbells using Monte Carlo simulations. Generally good agreement with reference literature data is obtained, which indicates that the SR-TI method is potentially very general and robust

Classical molecular dynamics simulation of microwave heating of liquids: the case of water

We perform a complete classical molecular dynamics study of the dielectric heating of water in the microwave region. MW frequencies ranging from 1.0 to 15.0 GHz are used together with a series of well-known empirical force fields. We show that the ability of an empirical force field to correctly predict the dielectric response of liquids to MW radiation should be evaluated on the basis of a joint comparison of the predicted and experimental static dielectric constant, frequency-dependent dielectric spectra, and heating profiles. We argue this is essential when multicomponent liquids are studied. We find the well-known SPCE and recently developed OPC3 empirical force fields of water are superior for reproducing dielectric properties, with the OPC3 force field particularly good for predicting heating rates at a range of MW frequencies

Self-referential Monte Carlo method for calculating the free energy of crystalline solids

A self-referential Monte Carlo method is described for calculating the free energy of crystalline solids. All Monte Carlo methods for the free energy of classical crystalline solids calculate the free-energy difference between a state whose free energy can be calculated relatively easily and the state of interest. Previously published methods employ either a simple model crystal, such as the Einstein crystal, or a fluid as the reference state. The self-referential method employs a radically different reference state; it is the crystalline solid of interest but with a different number of unit cells. So it calculates the free-energy difference between two crystals, differing only in their size. The aim of this work is to demonstrate this approach by application to some simple systems, namely, the face centered cubic hard sphere and Lennard-Jones crystals. However, it can potentially be applied to arbitrary crystals in both bulk and confined environments, and ultimately it could also be very efficient