Evaluation of B3LYP, X3LYP, and M06-class density functionals for predicting the binding energies of neutral, protonated, and deprotonated water clusters

In this paper we assess the accuracy of the B3LYP, X3LYP, and newly developed M06-L, M06-2X, and M06 functionals to predict the binding energies of neutral and charged water clusters including (H_2O)_n, n = 2−8, 20), H_3O+(H_2O_)n, n = 1−6, and OH−(H_2O)_n, n = 1−6. We also compare the predicted energies of two ion hydration and neutralization reactions on the basis of the calculated binding energies. In all cases, we use as benchmarks calculated binding energies of water clusters extrapolated to the complete basis set limit of the second-order Møller−Plesset perturbation theory with the effects of higher order correlation estimated at the coupled-cluster theory with single, double, and perturbative triple excitations in the aug-cc-pVDZ basis set. We rank the accuracy of the functionals on the basis of the mean unsigned error (MUE) between calculated benchmark and density functional theory energies. The corresponding MUE (kcal/mol) for each functional is listed in parentheses. We find that M06-L (0.73) and M06 (0.84) give the most accurate binding energies using very extended basis sets such as aug-cc-pV5Z. For more affordable basis sets, the best methods for predicting the binding energies of water clusters are M06-L/aug-cc-pVTZ (1.24), B3LYP/6-311++G(2d,2p) (1.29), and M06/aug-cc-PVTZ (1.33). M06-L/aug-cc-pVTZ also gives more accurate energies for the neutralization reactions (1.38), whereas B3LYP/6-311++G(2d,2p) gives more accurate energies for the ion hydration reactions (1.69)

Shock Induced Decomposition and Sensitivity of Energetic Materials by ReaxFF Molecular Dynamics

We develop strain-driven compression-expansion technique using molecular dynamics (MD) with reactive force fields (ReaxFF) to study the impact sensitivity of energetic materials. It has been applied to simulation of 1,3,5-trinitrohexahydro-s-triazine (RDX) crystal subjected to high-rate compression typical at the detonation front. The obtained results show that at lower compression ratio x = 1-V/V040%) all molecules decompose very quickly. We have observed both primary and secondary reactions during the decomposition process as well as production of various intermediates (NO2, NO, HONO, OH) and final products (H2O, N2, CO, CO2). The results of strain-driven compression-expansion modeling are in a good agreement with previous ReaxFF-MD shock simulations in RDX. Proposed approach might be useful for a quick test of sensitivity of energetic materials under conditions of high strain rate loading

Explicit Construction of Spin 4 Casimir Operator in the Coset Model SO^(5)1×SO^(5)m/SO^(5)1+m \hat{SO} (5)_{1} \times \hat{SO} (5)_{m} / \hat{SO} (5)_{1+m}

We generalize the Goddard-Kent-Olive (GKO) coset construction to the dimension 5/2 operator for $\hat{so} (5)$ and compute the fourth order Casimir invariant in the coset model $\hat{SO} (5)_{1} \times \hat{SO} (5)_{m} / \hat{SO} (5)_{1+m}$ with the generic unitary minimal $c < 5/2$ series that can be viewed as perturbations of the $m \rightarrow \infty$ limit, which has been investigated previously in the realization of $c= 5/2$ free fermion model.Comment: 11 page

Open ocean carbon monoxide photo-production

Sunlight-initiated photolysis of chromophoric dissolved organic matter (CDOM) is the dominant source of carbon monoxide (CO) in the open-ocean. A modelling study was conducted to constrain this source. Spectral solar irradiance was obtained from two models (GCSOLAR and SMARTS2). Water-column CDOM and total light absorption were modelled using spectra collected along a Meridional transect of the Atlantic ocean using a 200-cm pathlength liquid waveguide UV-visible spectrophotometer. Apparent quantum yields for the production of CO (AQYCO) from CDOM were obtained from a parameterisation describing the relationship between CDOM light absorption coefficient and AQYCO and the CDOM spectra collected. The sensitivity of predicted rates to variations in model parameters (solar irradiance, cloud cover, surface-water reflectance, CDOM and whole water light absorbance, and AQYCO was assessed. The model\u27s best estimate of open-ocean CO photoproduction was 47 +/- 7 Tg CO-C yr-1, with lower and upper limits of 38 and 84 Tg CO-C yr-1, as indicated by sensitivity analysis considering variations in AQYs, CDOM absorbance, and spectral irradiance. These results represent significant constraint of open-ocean CO photoproduction at the lower limit of previous estimates. Based on these results, and their extrapolation to total photochemical organic carbon mineralisation, we recommend a downsizing of the role of photochemistry in the open-ocean carbon cycle. (c) 2006 Elsevier Ltd. All rights reserved

Nitrous oxide emissions from the Arabian Sea: A synthesis

We computed high-resolution (1º latitude x 1º longitude) seasonal and annual nitrous oxide (N2O) concentration fields for the Arabian Sea surface layer using a database containing more than 2400 values measured between December 1977 and July 1997. N2O concentrations are highest during the southwest (SW) monsoon along the southern Indian continental shelf. Annual emissions range from 0.33 to 0.70 Tg N2O and are dominated by fluxes from coastal regions during the SW and northeast monsoons. Our revised estimate for the annual N2O flux from the Arabian Sea is much more tightly constrained than the previous consensus derived using averaged in-situ data from a smaller number of studies. However, the tendency to focus on measurements in locally restricted features in combination with insufficient seasonal data coverage leads to considerable uncertainties of the concentration fields and thus in the flux estimates, especially in the coastal zones of the northern and eastern Arabian Sea. The overall mean relative error of the annual N2O emissions from the Arabian Sea was estimated to be at least 65%

Nitrous oxide and methane in the Atlantic Ocean between 50 degrees North and 52 degrees South: Latitudinal distribution and sea-to-air flux

We discuss nitrous oxide (N2O) and methane (CH4) distributions in 49 vertical profiles covering the upper 300 m of the water column along two 13,500 km transects between 50°N and 52°S during the Atlantic Meridional Transect (AMT) programme (AMT cruises 12 and 13). Vertical N2O profiles were amenable to analysis on the basis of common features coincident with Longhurst provinces. In contrast, CH4 showed no such pattern. The most striking feature of the latitudinal depth distributions was a well-defined “plume” of exceptionally high N2O concentrations coincident with very low levels of CH4, located between 23.5°N and 23.5°S; this feature reflects the upwelling of deep waters containing N2O derived from nitrification, as identified by an analysis of N2O, apparent oxygen utilization (AOU) and NO3-, and presumably depleted in CH4 by bacterial oxidation. Sea-to-air emissions fluxes for a region equivalent to 42% of the Atlantic Ocean surface area were in the range 0.40–0.68 Tg N2O yr-1 and 0.81–1.43 Tg CH4 yr-1. Based on contemporary estimates of the global ocean source strengths of atmospheric N2O and CH4, the Atlantic Ocean could account for 6–15% and 4–13%, respectively, of these source totals. Given that the Atlantic Ocean accounts for around 20% of the global ocean surface, on unit area basis it appears that the Atlantic may be a slightly weaker source of atmospheric N2O than other ocean regions but it could make a somewhat larger contribution to marine-derived atmospheric CH4 than previously thought

Deformation and spallation of shocked Cu bicrystals with Σ3 coherent and symmetric incoherent twin boundaries

We perform molecular dynamics simulations of Cu bicrystals with two important grain boundaries (GBs), Σ3 coherent twin boundaries (CTB), and symmetric incoherent twin boundaries (SITB) under planar shock wave loading. It is revealed that the shock response (deformation and spallation) of the Cu bicrystals strongly depends on the GB characteristics. At the shock compression stage, elastic shock wave can readily trigger GB plasticity at SITB but not at CTB. The SITB can induce considerable wave attenuation such as the elastic precursor decay via activating GB dislocations. For example, our simulations of a Cu multilayer structure with 53 SITBs (∼1.5-μm thick) demonstrate a ∼80% elastic shock decay. At the tension stage, spallation tends to occur at CTB but not at SITB due to the high mobility of SITB. The SITB region transforms into a threefold twin via a sequential partial dislocation slip mechanism, while CTB preserves its integrity before spallation. In addition, deformation twinning is a mechanism for inducing surface step during shock tension stage. The drastically different shock response of CTB and SITB could in principle be exploited for, or benefit, interface engineering and materials design

Left-right loading dependence of shock response of (111)//(112) Cu bicrystals: Deformation and spallation

We investigate with molecular dynamics the dynamic response of Cu bicrystals with a special asymmetric grain boundary (GB), (111)//(112)〈110〉, and its dependence on the loading directions. Shock loading is applied along the GB normal either from the left or right to the GB. Due to the structure asymmetry, the bicrystals demonstrate overall strong left-right loading dependence of its shock response, including compression wave features, compression and tensile plasticity, damage characteristics (e.g., spall strength), effective wave speeds and structure changes, except that spallation remains dominated by the GB damage regardless of the loading directions. The presence or absence of transient microtwinning also depends on the loading directions

Role of anisotropy in the spin-dimer compound BaCuSi2O6

We present results of magnetisation and electron paramagnetic resonance experiments on the spin-dimer system BaCuSi2O6. Evidence indicates that the origin of anisotropic terms in the spin Hamiltonian is from magnetic dipolar interactions. Axial symmetry-breaking is on a very small energy scale of ~11 mK, confirming Bose Einstein condensation critical scaling over an extended temperature range in the vicinity of the quantum critical point.Comment: 4 pages, 4 figure

Energetic Materials at High Compression: First-Principles Density Functional Theory and Reactive Force Field Studies

We report the results of a comparative study of pentaerythritol tetranitrate (PETN) at high compression using classical reactive interatomic potential ReaxFF and first-principles density functional theory (DFT). Lattice parameters of PETN I, the ground state structure at ambient conditions, is obtained by ReaxFF and two different density functional methods (plane wave and LCAO pseudopotential methods) and compared with experiment. Calculated energetics and isothermal equation of state (EOS) upon hydrostatic compression obtained by DFT and ReaxFF are both in good agreement with available experimental data. Our calculations of the hydrostatic EOS at zero temperature are extended to high pressures up to 50 GPa. The anisotropic characteristics of PETN upon uniaxial compression were also calculated by both ReaxFF and DFT