Alauddin Ahmed

Solid-liquid equilibria and triple points of n-6 Lennard-Jones fluids

Molecular dynamics simulations are reported for the solid-liquid coexistence properties of n-6 Lennard-Jones fluids, where n=12, 11, 10, 9, 8, and 7. The complete phase behavior for these systems has been obtained by combining these data with vapor-liquid simulations. The influence of n on the solid-liquid coexistence region is compared using relative density difference and miscibility gap calculations. Analytical expressions for the coexistence pressure, liquid, and solid densities as a function of temperature have been determined, which accurately reproduce the molecular simulation data. The triple point temperature, pressure, and liquid and solid densities are estimated. The triple point temperature and pressure scale with respect to 1/n, resulting in simple linear relationships that can be used to determine the pressure and temperature for the limiting infinity-6 Lennard-Jones potential. The simulation data are used to obtain parameters for the Raveche, Mountain, and Streett and Lindemann melting rules, which indicate that they are obeyed by the n-6 Lennard Jones potentials. In contrast, it is demonstrated that the Hansen-Verlet freezing rule is not valid for n-6 Lennard-Jones potentials.

Effect of potential truncations and shifts on the solid-liquid phase coexistence of Lennard-Jones fluids

Molecular simulation results for the solid-liquid coexistence properties of untruncated, truncated, truncated and shifted, and truncated and shifted-force 12-6 Lennard-Jones potentials are reported. It is found that solid-liquid coexistence properties vary systematically with potential truncations, shifts, and cut-off values. Potential truncations and shifts have important consequences at low temperatures, particularly in the vicinity of the triple point. The main influence is on the coexistence pressure whereas both liquid and solid densities are less sensitive to the truncations and shifts. The data reported in this work indicate that the cut-off radius mainly affects the properties of the liquid phase whereas its influence on the solid phase is almost negligible. The data suggest a monotonic variation of the melting temperature as a function of cut-off radius, which contradicts the oscillatory behavior of the melting temperature reported elsewhere.

Solid-liquid phase equilibria of the Gaussian core model fluid

The solid-liquid phase equilibria of the Gaussian core model are determined using the GWTS [J. Ge, G.-W. Wu, B. D. Todd, and R. J. Sadus, J. Chem. Phys. 119, 11017 (2003)] algorithm, which combines equilibrium and nonequilibrium molecular dynamics simulations. This is the first reported use of the GWTS algorithm for a fluid system displaying a reentrant melting scenario. Using the GWTS algorithm, the phase envelope of the Gaussian core model can be calculated more precisely than previously possible. The results for the low-density and the high-density (reentrant melting) sides of the solid state are in good agreement with those obtained by Monte Carlo simulations in conjunction with calculations of the solid free energies. The common point on the Gaussian core envelope, where equal-density solid and liquid phases are in coexistence, could be determined with high precision.

Physicochemical Properties of Hazardous Energetic Compounds from Molecular Simulation.

A protocol is presented and used for the computation of physicochemical properties of nitroaromatic energetic compounds (ECs) using molecular simulation. Solvation and self-solvation free energies of ECs are computed using an expanded ensemble (EE) molecular dynamics method, with the TraPPE-UA/CHELPG and CGenFF/CHELPG force field models. Thermodynamic pathways relating Gibbs free energies and physicochemical properties are used to predict the room temperature vapor pressures, solubilities (in water and 1-octanol), Henrys law constants, and partition coefficients (octanol-water, air-water, and air-octanol) for liquid, subcooled, and solid ECs from the molecular simulations. These predictions are compared to experimental data where available. It is found that the use of the TraPPE-UA model with CHELPG charges computed here leads to predictions of measured physicochemical properties of comparable accuracy to that of other theoretical and empirical models. However, the advantage of the method used here is that with no experimental data, unlike other methods, a number of physicochemical properties for a compound can be calculated from only its atomic connectivity, charges obtained from density function theory (DFT), and choice of force field using two simulations: its self-solvation free energy and its Gibbs free energy in a solvent.

Solvation free energies and hydration structure of N-methyl-p-nitroaniline

Solvation Gibbs energies of N-methyl-p-nitroaniline (MNA) in water and 1-octanol are calculated using the expanded ensemble molecular dynamics method with a force field taken from the literature. The accuracy of the free energy calculations is verified with the experimental Gibbs free energy data and found to reproduce the experimental 1-octanol∕water partition coefficient to within ±0.1 in log unit. To investigate the hydration structure around N-methyl-p-nitroaniline, an independent NVT molecular dynamics simulation was performed at ambient conditions. The local organization of water molecules around the solute MNA molecule was investigated using the radial distribution function (RDF), the coordination number, and the extent of hydrogen bonding. The spatial distribution functions (SDFs) show that the water molecules are distributed above and below the nitrogen atoms parallel to the plane of aromatic ring for both the methylamino and nitro functional groups. It is found that these groups have a significant effect on the hydration of MNA with water molecules forming two weak hydrogen bonds with both the methylamino and nitro groups. The hydration structures around the functional groups in MNA in water are different from those that have been found for methylamine, nitrobenzene, and benzene in aqueous solutions, and these differences together with weak hydrogen bonds explain the lower solubility of MNA in water. The RDFs together with SDFs provide a tool for the understanding the hydration of MNA (and other molecules) and therefore their solubility.

Hydration Free Energies of Multifunctional Nitroaromatic Compounds

Nitroaromatic compounds (NACs) are used as energetic materials, reagents, and pesticides; however, they are potentially hazardous for the environment and human health. To predict the environmental distribution of these compounds, the vapor pressure, aqueous solubility, and Henrys law constant are important properties, as is the solvation free energy in water from which the latter two can be computed. Here, we have calculated the hydration free energies for a set of nine nitroaromatic compounds containing one, two, and three nitro groups using the expanded ensemble molecular dynamics simulation method with TIP3P water and the GAFF, CGenFF, OPLS-AA, and TraPPE force field parameters and the RESP (gas phase), CHELPG (gas phase), and CM4 (aqueous phase) partial atomic charges calculated here. Also, we have computed hydration free energies using the reported default partial atomic charges of the OPLS-AA force field and using the semiempirical AM1-BCC charges with GAFF parameters. The effect of water model flexibility on the computation of hydration free energy is examined with CGenFF/(CHELPG+SPC-Fw) model. All the force fields studied generally led to less accurate predictions with increasing numbers of nitro groups. The average unsigned errors (AUE) show that 6 of 16 force-field/(charge+water) models used perform approximately equally well in predicting measured hydration free energies: these are CGenFF/(CHELPG+TIP3P), CGenFF/(CM4+TIP3P), OPLS-AA/(CHELPG+TIP3P), OPLS-AA/(CM4+TIP3P), TraPPE-UA/(CHELPG+TIP3P), and TraPPE-UA/(CM4+TIP3P). When using the default atomic charges, the OPLS-AA force field was the most accurate, though using CHELPG and CM4 charges led to better predictions. Our analyses indicate that not only the charges but also the van der Waals interaction parameters for the nitro-group nitrogen and oxygen atoms in the force fields are partly responsible for the performance variations in predicting solvation free energies. We also compared the force field-based simulation results with the predictions from the SM6 solvation model and Abraham linear solvation energy relationship (LSER) method. With an appropriate choice of theory and basis set both for geometry optimization and computation, which unfortunately is not known a priori, the SM6 model hydration free energy predictions for the NACs are comparable to the simulation results here. The Abraham LSER predictions with descriptors obtained from the Platts method are of reasonable accuracy. A useful addition to this paper is the Supporting Information that contains a compiled and evaluated list of the hydration free energies of the NACs studied here assembled from the literature.

Erratum: Solid-liquid equilibria and triple points of n-6 Lennard-Jones fluids [J. Chem. Phys. 131, 174504 (2009)]

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