Peter Wind

A multi‐model study of the hemispheric transport and deposition of oxidised nitrogen

Fifteen chemistry-transport models are used to quantify, for the first time, the export of oxidised nitrogen (NOy) to and from four regions (Europe, North America, South Asia, and East Asia), and to estimate the uncertainty in the results. Between 12 and 24% of the NOx emitted is exported from each region annually. The strongest impact of each source region on a foreign region is: Europe on East Asia, North America on Europe, South Asia on East Asia, and East Asia on North America. Europe exports the most NOy, and East Asia the least. East Asia receives the most NOy from the other regions. Between 8 and 15% of NOx emitted in each region is transported over distances larger than 1000 km, with 3–10% ultimately deposited over the foreign regions.

Is regional air quality model diversity representative of uncertainty for ozone simulation

We examine whether seven state-of-the-art European regional air quality models provide daily ensembles of predicted ozone maxima that encompass observations. Using tools borrowed from the evaluation of ensemble weather forecasting, we analyze statistics of simulated ensembles of ozone daily maxima over an entire summer season. Although the model ensemble overestimates ozone, the distribution of simulated concentrations is representative of the uncertainty. The spread of simulations is due to random fluctuations resulting from differences in model formulations and input data, but also to the spread between individual model systematic biases. The ensemble average skill increases as the spread decreases. The skill of the ensemble in giving probabilistic predictions of threshold exceedances is also demonstrated. These results allow for optimism about the ability of this ensemble to simulate the uncertainty of the impact of emission control scenarios.

Origin of the linear dependence with atomic number of correlation energies in neutral atoms

Using only the ground-state electron density, an interpretation is proposed, and supported by numerical calculations on the Kr and Xe atoms, for the approximately linear dependence of the correlation energies of neutral atoms with the nuclear charge Z. The results of the Colle-Salvetti correlation energy are in quantitative agreement with experiment in the range of nuclear charge Z ˇ- 18. The scaling of the Thomas-Fermi density with Z is invoked for larger values of Z.

A theoretical study of the (H2)2 dimer. II. The potential energy surface

Abstract An extended geminal model has been applied to calculate the potential energy surface of (H 2 ) 2 within the rigid rotor approximation. By adopting a [8s, 4p, 2d] uncontracted Gaussian type basis set and the numerical model EXRHF3, the total electronic energy has been calculated for 16 different internuclear distances from 3 to 12 au, and 7 relative orientations for each distance. An isotropic well depth of 98.3 μhartree is obtained. The most reliable experimental estimate of the well depth is 108 μhartree. An estimaton of the errors due to basis set superposition error, truncation erros related to the electron correlation approach, adoption of an incomplete basis set, and the rigid rotor approximation, yields roughly 6 μhartree. It is argued that the residual difference, between the semi-experimental potentials and the ab initio potential is likely to have its origin in the regularising assumption used for the former.

A theoretical study of the (H2)2 dimer. III. The isotropic potential

Abstract An extended geminal model has been applied to calculate the isotropic potential of the (H2)2 dimer within the rigid rotor approximation. By adopting a [6s,4p,3d,2f] contracted Gaussian type basis set and the numerical model EXRHF3, 16 different distances from 3 to 12 au are considered. The isotropic potential is calculated directly by approximating it by the value of the potential for one particular orientation of the molecule. It is demonstrated that this last approximation induces an error of less than 1%. The calculated isotropic potential presents a minimum of 108.9 μhartree at a distance of 6.58 au, in close agreement with experimentally based results.

A theoretical study of the (H2)2 dimer. I: Bonding

Abstract An extended geminal model has been applied to analyse the bonding of the (H2)2 dimer. By adopting a [8s, 4p, 2d] uncontracted Gaussian type basis set, and the numerical model EXRHF3, the T structure and the linear structure of (H2)2 has been studied in detail. The changes in the one-electron density during formation of the complex, are analysed in terms of charge centroids and charge ellipsoids of the (localized) geminal densities. The intermolecular potential is decomposed into a sum of distortion energies for the subsystems and the interaction energy between the distorted subsystems. The latter term is decomposed into a Coulombic, exchange and correlation part. The Coulombic component is further partitioned into an electrostatic and induction component. By considering seven different structures of the complex it is found that the ordering of the values of the electrostatic energy component coincides with the ordering of the values of the intermolecular potential. In particular both the electrostatic energy component and the intermolecular potential predict that the global minimum of the potential has a T structure.

Linear scaling Coulomb interaction in the multiwavelet basis, a parallel implementation

We present a parallel and linear scaling implementation of the calculation of the electrostatic potential arising from an arbitrary charge distribution. Our approach is making use of the multi-resolution basis of multiwavelets. The potential is obtained as the direct solution of the Poisson equation in its Greens function integral form. In the multiwavelet basis, the formally non local integral operator decays rapidly to negligible values away from the main diagonal, yielding an effectively banded structure where the bandwidth is only dictated by the requested accuracy. This sparse operator structure has been exploited to achieve linear scaling and parallel algorithms. Parallelization has been achieved both through the shared memory (OpenMP) and the message passing interface (MPI) paradigm. Our implementation has been tested by computing the electrostatic potential of the electronic density of long-chain alkanes and diamond fragments showing (sub)linear scaling with the system size and efficent parallelization.

Application of the EMEP Unified Model to the UK with a Horizontal Resolution of 5 × 5 km2

The EMEP Unified model (Simpson et al. 2003; http://www.emep.int) is an Eulerian model that is driven by real-time meteorology. The model is applied over Europe for multiple years on a 50 × 50 km grid, with meteorological fields updated every 3 h. While comparisons with measurements have shown generally robust performance of the EMEP model on a European scale (e.g. Simpson et al. 2006), pollutants such as reactive nitrogen and sulphur have a high spatial variability in their emissions and a short life time. Therefore, the associated dry deposition also has a high spatial variability (Vieno 2006; van Pul et al. 2009, this volume). This is very important when critical loads of nitrogen are calculated for specific ecosystems. For this reason a number of models have been developed for high resolution operation at a national scale. To address these issues for the UK, the EMEP Unified Model is being developed, using a nested approach. This model application, referred to as EMEP4UK, has been developed at a 5 × 5 km resolution covering the whole of the British Isles. By comparison with existing statistical models of atmospheric chemistry and transport over the UK (e.g. Singles et al. 1998; Lee et al. 2000; Metcalfe et al. 2001; Fournier et al. 2005; Vieno 2006; Dore et al. 2007), the EMEP4UK model therefore has the advantage of using real time meteorology, enabling the interactions between emissions, meteorology, concentrations and deposition to be addressed at a fine spatial scale.

Ab initio calculation of three-body interaction in the (H2)3 trimer

Abstract The non-additivity in the (H 2 ) 3 trimer is calculated at different geometries using an extended geminal model. The isotropic part of the three-body non-additive potential energy is derived. It is shown that the three-body potential is converging relatively rapidly with respect to the size of the basis set. With a 6s,3p,2d basis, near the equilibrium geometry, the global error of the three-body potential is estimated to be of only 7%. At large distances the results are in accordance with the Axilrod-Teller-Muto triple-dipole approximation.

Theoretical studies of the phase transition in the anisotropic two-dimensional square spin lattice

The phase transition occurring in a square 2-D spin lattice governed by an anisotropic Heisenberg Hamiltonian has been studied according to two recently proposed methods. The first one, the Dressed Cluster Method, provides excellent evaluations of the cohesive energy, the discontinuity of its derivative around the critical (isotropic) value of the anisotropy parameter confirms the first-order character of the phase transition. Nevertheless the method introduces two distinct reference functions (either N\\eel or XY) which may in principle force the discontinuity. The Real Space Renormalization Group with Effective Interactions does not reach the same numerical accuracy but it does not introduce a reference function and the phase transition appears qualitatively as due to the existence of two domains, with specific fixed points. The method confirms the dependence of the spin gap on the anisotropy parameter occurring in the Heisenberg-Ising domain.