First-Principles-Based Dispersion Augmented Density Functional Theory: From Molecules to Crystals

Standard implementations of density functional theory (DFT) describe well strongly bound molecules and solids but fail to describe long-range van der Waals attractions. We propose here first-principles-based augmentation to DFT that leads to the proper long-range 1/R^6 attraction of the London dispersion while leading to low gradients (small forces) at normal valence distances so that it preserves the accurate geometries and thermochemistry of standard DFT methods. The DFT-low gradient (DFT-lg) formula differs from previous DFT-D methods by using a purely attractive dispersion correction while not affecting valence bond distances. We demonstrate here that the DFT-lg model leads to good descriptions for graphite, benzene, naphthalene, and anthracene crystals, using just three parameters fitted to reproduce the full potential curves of high-level ab initio quantum mechanics [CCSD(T)] on gas-phase benzene dimers. The additional computational costs for this DFT-lg formalism are negligible

Microscopic mechanism of water diffusion in glucose glasses

The preservation of biomaterials depends critically on the mobility of water in the glassy state, manifested as a secondary beta relaxation and diffusion. We use coarse grain simulations to elucidate the molecular mechanism underlying the relaxations for water-glucose glass, finding two pathways for water diffusion: (i) water jumps into neighbor water positions (linking to water structure), and (ii) water jumps into glucose positions (coupling to glucose rotation). This work suggests strategies for enhancing preservation by stiffening the segmental motions of the carbohydrates

Understanding β-Hydride Eliminations from Heteroatom Functional Groups

Using density functional theory, we investigated detailed aspects of the quintessential organometallic process, β-hydride elimination (BHE). In general, we find that most BHE processes from alkyl functional group β-atoms are facile, while BHE processes from heteroatom functional groups (N and O) are prohibitively high in energy. We present calculated molecular orbitals and atomic NBO charges obtained from snapshots along reaction profiles to present a qualitative overview for how heteroatoms adversely affect these processes. We discuss these results to provide an illustration for how these processes proceed, clarifying a sometimes oversimplified model for these reactions

Shouldering in B diffusion profiles in Si: Role of di-boron diffusion

The role of di-boron diffusion in evolution of B diffusion profiles has been investigated. We find that boron pair (B-s-B-i) diffusion can become as important as boron-interstitial pair (B-s-Si-i) diffusion when both boron concentration and annealing temperature are very high, leading to concentration-dependent B diffusion. Our simulated B diffusion profiles with dramatic shouldering are in excellent agreement with experimental ones reported by Schroer [Appl. Phys. Lett. 74, 3996 (1999)] for high-temperature (approximate to 1200 degrees C) postimplantion annealing of ultralow-energy (approximate to500 eV) implanted high-concentration (>10(19) cm(-3)) boron in silicon

High H2 Storage of Hexagonal Metal−Organic Frameworks from First-Principles-Based Grand Canonical Monte Carlo Simulations

Stimulated by the recent report by Yaghi and co-workers of hexagonal metal−organic frameworks (MOF) exhibiting reversible binding of up to 7.5 wt % at 77 K and 70 bar for MOF-177 (called here IRMOF-2-24), we have predicted additional trigonal organic linkers, including IRMOF-2-60, which we calculate to bind 9.7 wt % H2 storage at 77 K and 70 bar, the highest known value for 77 K. These calculations are based on grand canonical Monte Carlo (GCMC) simulations using force fields that match accurate quantum mechanical calculations on the binding of H2 to prototypical systems. These calculations were validated by comparison to the experimental loading curve for IRMOF-2-24 at 77K. We then used the theory to predict the effect of doping Li into the hexagonal MOFs, which leads to substantial H2 density even at ambient temperatures. For example, IRMOF-2-96-Li leads to 6.0 wt % H2 storage at 273 K and 100 bar, the first material to attain the 2010 DOE target

Catalytic role of boron atoms in self-interstitial clustering in Si

Using density functional theory (DFT) calculations and kinetic simulations, we have investigated the influence of boron atoms on self-interstitial clustering in Si. From DFT calculations of neutral interstitial clusters with a single B atom (BsIn, nIn–1 + BsI) becomes substantially weaker than that of an interstitial (BsIn-->BsIn–1 + I) when n>=4. This implies boron can be liberated while leaving an interstitial cluster behind. Our kinetic simulations including the boron liberation explain well experimental observations reported by J. L. Benton et al., J. Appl. Phys. 82, 120 (1997)

Excited Electron Dynamics Modeling of Warm Dense Matter

We present a model (the electron force field, or eFF) based on a simplified solution to the time-dependent Schrödinger equation that with a single approximate potential between nuclei and electrons correctly describes many phases relevant for warm dense hydrogen. Over a temperature range of 0 to 100 000 K and densities up to 1 g/cm^3, we find excellent agreement with experimental, path integral Monte Carlo, and linear mixing equations of state, as well as single-shock Hugoniot curves from shock compression experiments. In principle eFF should be applicable to other warm dense systems as well