The fate of chemical and radioactive wastes in the environment is related to the ability of natural phases to attenuate and immobilize contaminants through chemical sorption and precipitation processes. Our understanding of these complex processes at the atomic level is provided by a few experimental and analytical methods such as X-ray absorption and NMR spectroscopies. However, due to complexities in the structure and composition of clay and other hydrated minerals, and the inherent uncertainties of the experimental methods, it is important to apply theoretical molecular models for a fundamental atomic-level understanding, interpretation, and prediction of these phenomena. In this effort, we have developed a general force field, CLAYFF, suitable for the simulation of hydrated and multicomponent mineral systems and their interfaces with aqueous solutions. Interatomic potentials were derived from parametrizations incorporating structural and spectroscopic data for a variety of simple hydrated compounds. A flexible SPC-based water model is used to describe the water and hydroxyl behavior. Metal-oxygen interactions are described by a LennardJones function and a Coulombic term with partial charges derived by Mulliken and ESP analysis of DFT results. Bulk structures, relaxed surface structures, and intercalation processes are evaluated and compared to experimental and spectroscopic findings for validation. Our approach differs from most others in that we treat most interatomic interactions as nonbonded. This allows us to effectively use the force field for a wide variety of phases and to properly account for energy and momentum transfer between the fluid phase and the solid, while keeping the number of parameters small enough to allow modeling of relatively large and highly disordered systems. Simulations of clay, hydroxide, and oxyhydroxide phases and their interfaces with aqueous solutions combine energy minimization and molecular dynamics methods to describe the structure and behavior of water, hydroxyl, surface species, and intercalates in these systems. The results obtained to date demonstrate that CLAYFF shows good promise to evolve into a widely adaptable and broadly effective force field for molecular simulations of fluid interfaces with clays and other clay-related phases, as well as other inorganic materials characterized by complex, disordered, and often ill-determined structure and composition.

Oligoacenes form a fundamental class of polycyclic aromatic hydrocarbons (PAH) which have been extensively explored for use as organic (semi) conductors in the bulk phase and thin films. For this reason it is important to understand their electronic properties in the condensed phase. In this investigation, we use density functional theory with Tkatchenko-Scheffler dispersion correction to explore several crystalline oligoacenes (naphthalene, anthracene, tetracene, and pentacene) under pressures up to 25 GPa in an effort to uncover unique electronic/optical properties. Excellent agreement with experiment is achieved for the pressure dependence of the crystal structure unit cell parameters, densities, and intermolecular close contacts. The pressure dependence of the band gaps is investigated as well as the pressure induced phase transition of tetracene using both generalized gradient approximated and hybrid functionals. It is concluded that none of the oligoacenes investigated become conducting under elevated pressures, assuming that the molecular identity of the system is maintained.

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Abstract:A new forcefield model was developed for modeling phosphate materials that many important applications in the electronics and biomedical industries. Molecular have dynamics simulations of a series of lithium phosphate glass compositions were performed using the new forcefield model. A high concentration of three member rings (P303) was found in the glass of intermediate composition (0.2 LizO . 0.8 P205) that corresponds to the minimum in the glass transition temperature curve for the compositional series.

We show that electrodynamic dipolar interactions, responsible for long-range fluctuations in matter, play a significant role in the stability of molecular crystals. Density functional theory calculations with van der Waals interactions determined from a semilocal "atom-in-a-molecule" model result in a large overestimation of the dielectric constants and sublimation enthalpies for polyacene crystals from naphthalene to pentacene, whereas an accurate treatment of nonlocal electrodynamic response leads to an agreement with the measured values for both quantities. Our findings suggest that collective response effects play a substantial role not only for optical excitations, but also for cohesive properties of noncovalently bound molecular crystals. Polyacene molecular crystals form a fundamental class of aromatic solids, and have been extensively studied as potential materials for organic electronics. [1][2][3] It is understood that the optical properties of polyacenes are very sensitive to longrange intra-and intermolecular electrodynamic interactions. This is reflected by shifts in the optical absorption frequencies upon increasing the molecule size or upon solid formation, 4 and is further exhibited by the visible color of oligoacene crystals, which changes from transparent in naphthalene and anthracene, to bright orange in tetracene, and deep blue in pentacene. 4,5 The optical absorption spectrum is directly related to the polarizability through the Kramers-Kronig transformation.6 Therefore, the observed changes in the optical spectrum upon crystallization of polyacenes are accompanied by a change in the molecular polarizability. In addition, these changes in polarization should directly impact the crystal lattice energy. However, the effect of electrodynamic intermolecular interactions on the cohesive properties of molecular crystals remains poorly understood. In this Rapid Communication, we show that the dipolar electrodynamic coupling between polyacene molecules reduces the solid dielectric constant by 15%, and has an impact of up to 0.5 eV per molecule on the computed van der Waals (vdW) energies and sublimation enthalpies of these molecular crystals. Our results imply that electrodynamic response is crucial for describing both the cohesive energy and the optical properties of molecular crystals, also providing strong quantitative support to empirical relations between stability and refractive index of molecular crystals. 7Polyacene crystals are extended aromatic networks characterized by polarizable π clouds. Therefore, an appreciable part of the crystal lattice energy stems from ubiquitous attractive vdW dispersion interactions. When studying the cohesion of molecular systems, for example, using densityfunctional theory (DFT) 8,9 or classical potentials, 10 the vdW energy is typically computed using effective polarizabilities for hybridized "atoms" inside a molecule. It is common to approximate the frequency-dependent polarizability of every atom using a single effective excitation frequency (also called t...

The quest for cheap, light, flexible materials for use in electronic applications has resulted in the exploration of soft organic materials as possible candidates, and several polycyclic aromatic hydrocarbons (PAH) have been shown to be versatile (semi) conductors. In this investigation, dispersion inclusive density functional theory is used to explore all of the current crystalline PAHs within the Cambridge Structure Database (CSD) from both structural and electronic standpoints. Agreement is achieved between the experimental and calculated crystalline structures as well as the electronic properties: Specifically, variation between the mass densities, unit cell parameters and intermolecular close contact fractions were within +5%, ±2%, and ±1% of experiment, respectively. It is found that a simple addition of a ~1 eV constant to the DFT-PBE electronic band gaps provides good agreement with the experimental optical gaps of both gas phase (within ±2.6%) and crystalline (within ±3.5%) PAHs. Structural and electronic analysis revealed several correlations/trends; where ultimately, limits in the band gaps as a function of structure are established. Finally, analysis of the difference between band gaps of the isolated molecules and crystals (ΔE_g^Xtal-Mol) demonstrates that ΔE_g^Xtal-Mol can be captured qualitatively by PBE and PBE0 functionals, yet significant quantitative deviations remain between these functionals and experiment

The density functional theory (DFT) method was used to study the effect of nanoconfinement on the energetics of Mg-MgH2 systems. Varying levels of loading of the Mg/MgH2 particles into a (10,10) carbon nanotube were examined, and the corresponding energetics were computed. A clear trend was observed that, as the level of loading increases (increasing confinement), the net energy change in the hydrogen sorption/desorption processes decreases to a significant level when the loading approaches the maximum. The confinement was found not to depend on the tube length of the confining nanotubes.

Prediction of the relative stabilities and phase transition behavior of molecular crystalline polymorphs is highly coveted as distinct phases can possess different physical and chemical properties while having similar energies. Crystalline tetracyanoethylene (TCNE, C 6 N 4 ) is known to exhibit rich solid state phase behavior under different thermodynamic conditions, as demonstrated by a wealth of experimental studies on this system. Despite this fact, the role of temperature and kinetics on the phase diagram of TCNE remains poorly understood. Here, first-principles calculations and highresolution Fourier-transformed infrared (HR-FTIR) spectroscopy experiments are used to study the relative stabilities of the cubic and monoclinic phases of TCNE as a function of temperature. Specifically, density-functional theory with the van der Waals interactions method of Tkatchenko and Scheffler (DFT+vdW) is employed. The accuracy of this approach is demonstrated by the excellent agreement between the calculated and experimental structures. We find that the cubic phase is the most stable polymorph at 0 K, but becomes less favorable than the monoclinic phase at 160 K. This temperature-induced phase transition is explained on the basis of varying close contacts and vibrational entropies as a function of temperature. These findings are supported by a temperaturedependent HR-FTIR linewidth study of the CMN vibrons.