Molecular dynamics simulations of uranyl and plutonyl cations in a task-specific ionic liquid.

Ionic liquids (ILs) are a unique class of solvents with potential applications in advanced separation technologies relevant to the nuclear industry. ILs are salts with low melting points and a wide range of tunable physical properties, such as viscosity, hydrophobiciy, conductivity, and liquidus range. ILs have negligible vapor pressure, are often non-flammable, and can have high thermal stability and a wide electrochemical window, making them attractive for use in separations processes relevant to the nuclear industry.

First-Principles Simulations of CuCl in High-Temperature Water Vapor.

Experimental data suggest that the solubility of copper in high-temperature water vapor is controlled by the formation of hydrated clusters of the form CuCl(HO), where the average number of water molecules in the cluster generally increases with increasing density [Migdisov, A. A.; et al.

Thermodynamics, dynamics, and structure of supercritical water at extreme conditions.

Molecular dynamics (MD) simulations to understand the thermodynamic, dynamic, and structural changes in supercritical water across the Frenkel line and the melting line have been performed. The two-phase thermodynamic model [J. Phys.

Electrical conductivity, ion pairing, and ion self-diffusion in aqueous NaCl solutions at elevated temperatures and pressures.

We have performed classical molecular dynamics (MD) simulations of aqueous sodium chloride (NaCl) solutions from 298 to 674 K at 200 bars to understand the influence of ion pairing and ion self-diffusion on electrical conductivity in high-temperature/high-pressure salt solutions. Conductivity data obtained from the MD simulation highlight an apparent anomaly, namely, a conductivity maximum as temperature increases along an isobar, which has been also observed in experimental studies. By examining both velocity autocorrelation and cross-correlation terms of the Green-Kubo integral, we quantitatively demonstrate that the conductivity anomaly arises mainly from a competition between the single-ion self-diffusion and the contact ion pair formation.

Synthesis, Characterization, and Density Functional Theory Analysis of Uranium and Thorium Complexes Containing Nitrogen-Rich 5-Methyltetrazolate Ligands.

Two nitrogen-rich, isostructural complexes of uranium and thorium, (C5Me5)2U[η(2)-(N,N')-tetrazolate]2 (7) and (C5Me5)2Th[η(2)-(N,N')-tetrazolate]2 (8), containing 5-methyltetrazolate, have been synthesized and structurally characterized by single-crystal X-ray diffraction, electrochemical methods, UV-visible-near-IR spectroscopy, and variable-temperature (1)H NMR spectroscopy. Density functional theory (DFT) calculations yield favorable free energies of formation (approximately -375 kJ/mol) and optimized structures in good agreement with the experimental crystal structures. Additionally, calculated NMR chemical shifts of 7 and 8 are in good agreement with the variable-temperature (1)H NMR experiments.

Structure and dynamics of uranyl(VI) and plutonyl(VI) cations in ionic liquid/water mixtures via molecular dynamics simulations.

A fundamental understanding of the behavior of actinides in ionic liquids is required to develop advanced separation technologies. Spectroscopic measurements indicate a change in the coordination of uranyl in the hydrophobic ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf2N]) as water is added to the system. Molecular dynamics simulations of dilute uranyl (UO2(2+)) and plutonyl (PuO2(2+)) ) solutions in [EMIM][Tf2N]/water mixtures have been performed in order to examine the molecular-level coordination and dynamics of the actinyl cation (AnO2(2+)) ); An = U, Pu) as the amount of water in the system changes.

Examining the phase transition behavior of amphiphilic lipids in solution using statistical temperature molecular dynamics and replica-exchange Wang-Landau methods.

Two different techniques - replica-exchange Wang-Landau (REWL) and statistical temperature molecular dynamics (STMD) - were applied to systematically study the phase transition behavior of self-assembling lipids as a function of temperature using an off-lattice lipid model. Both methods allow the direct calculation of the density of states with improved efficiency compared to the original Wang-Landau method. A 3-segment model of amphiphilic lipids solvated in water has been studied with varied particle interaction energies (ε) and lipid concentrations.

Incorporating configurational-bias Monte Carlo into the Wang-Landau algorithm for continuous molecular systems.

Configurational-bias Monte Carlo has been incorporated into the Wang-Landau method. Although the Wang-Landau algorithm enables the calculation of the complete density of states, its applicability to continuous molecular systems has been limited to simple models. With the inclusion of more advanced sampling techniques, such as configurational-bias, the Wang-Landau method can be used to simulate complex chemical systems.

A Wang-Landau study of a lattice model for lipid bilayer self-assembly.

The Wang-Landau (WL) Monte Carlo method has been applied to simulate the self-assembly of a lipid bilayer on a 3D lattice. The WL method differs from conventional Monte Carlo methods in that a complete density of states is obtained directly for the system, from which properties, such as the free energy, can be derived. Furthermore, from a single WL simulation, continuous curves of the average energy and heat capacity can be determined, which provide a complete picture of the phase behavior.

Electrostatically embedded many-body method for dipole moments, partial atomic charges, and charge transfer.

Fragment methods have been widely studied for computing energies and forces, but less attention has been paid to nonenergetic properties. Here we extend the electrostatically embedded many-body (EE-MB) method to the calculation of cluster dipole moments, dipole moments of molecules in clusters, partial atomic charges, and charge transfer, and we test and validate the method by comparing to results calculated for the entire system without fragmentation. We also compare to calculations carried out by the conventional many-body (MB) method without electrostatic embedding.

Transferable potentials for phase equilibria-coarse-grain description for linear alkanes.

Coarse-grain potentials allow one to extend molecular simulations to length and time scales beyond those accesssible to atomistic representations of the interacting system. Since the coarse-grain potentials remove a large number of interaction sites and, hence, a large number of degrees of freedom, it is generally assumed that coarse-grain potentials are not transferable to different systems or state points (temperature and pressure). Here we apply lessons learned from the parametrization of transferable atomistic potentials to develop a systematic procedure for the parametrization of transferable coarse-grain potentials.

Effects of an applied electric field on the vapor-liquid equilibria of water, methanol, and dimethyl ether.

Gibbs ensemble Monte Carlo simulations are employed to examine the influence of moderately strong electric fields on the vapor-liquid coexistence curves and on structural and energetic properties of the saturated phases of water, methanol, and dimethyl ether. The application of an electric field of 0.1 V/A increases the critical temperature and normal boiling point by approximately 3% compared to the zero field case for all three compounds, whereas the critical density is found to decrease by 1% for methanol and dimethly ether and by 3% for water.

Monte Carlo simulations of binary mixtures of nitrotoluene isomers with n-decane.

Previous experimental studies [Sliwinska-Bartkowiak et al., Chem. Phys.

TraPPE-UA force field for acrylates and Monte Carlo simulations for their mixtures with alkanes and alcohols.

An extension of the transferable potentials for phase equilibria-united atom (TraPPE-UA) force field to acrylate and methacrylate monomers is presented. New parameters were fit to the liquid density, normal boiling point, saturated vapor pressure, and (where experimentally available) critical constants of 1,3-butadiene, isoprene, methyl acrylate, and methyl methacrylate using Gibbs ensemble Monte Carlo simulations. Excellent agreement with experiment was obtained for the parametrization compounds and seven additional acrylate and methacrylate compounds, with average errors in liquid density and normal boiling point of approximately 1%.

Self-consistent polarization density functional theory: application to argon.

We present a comprehensive set of results for argon, a case study in weak interactions, using the self-consistent polarization density functional theory (SCP-DFT). With minimal parametrization, SCP-DFT is found to give excellent results for the dimer interaction energy, the second virial coefficient, the liquid structure, and the lattice constant and cohesion energy of the face-centered cubic crystal compared to both accurate theoretical and experimental benchmarks. Thus, SCP-DFT holds promise as a fast, efficient, and accurate method for performing ab initio dynamics that include additional polarization and dispersion interactions for large, complex systems involving solvation and bond breaking.