3,632 research outputs found
Extension of molecular electronic structure methods to the solid state: computation of the cohesive energy of lithium hydride
No full textWe describe a simple strategy for calculating the cohesive energy of certain kinds of crystal using readily available quantum chemistry techniques. The strategy involves the calculation of the electron correlation energies of a hierarchy of free clusters, and the cohesive energy E(coh) is extracted from the constant of proportionality between these correlation energies and the number of atoms in the limit of large clusters. We apply the strategy to the LiH crystal, using the MP2 and CCSD(T) schemes for the correlation energy, and show that for this material E(coh) can be obtained to an accuracy of similar to 30 meV per ion pair. Comparison with the experimental value, after correction for zero- point energy, confirms this accuracy
Assessing the accuracy of quantum Monte Carlo and density functional theory for energetics of small water clusters
No full textWe present a detailed study of the energetics of water clusters (H2O)(n) with n <= 6, comparing diffusion Monte Carlo (DMC) and approximate density functional theory (DFT) with well converged coupled-cluster benchmarks. We use the many-body decomposition of the total energy to classify the errors of DMC and DFT into 1-body, 2-body and beyond-2-body components. Using both equilibrium cluster configurations and thermal ensembles of configurations, we find DMC to be uniformly much more accurate than DFT, partly because some of the approximate functionals give poor 1-body distortion energies. Even when these are corrected, DFT remains considerably less accurate than DMC. When both 1- and 2-body errors of DFT are corrected, some functionals compete in accuracy with DMC; however, other functionals remain worse, showing that they suffer from significant beyond-2-body errors. Combining the evidence presented here with the recently demonstrated high accuracy of DMC for ice structures, we suggest how DMC can now be used to provide benchmarks for larger clusters and for bulk liquid water. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4730035
Calculation of properties of crystalline lithium hydride using correlated wave function theory
No full textThe lattice parameter, bulk modulus, and cohesive energy of lithium hydride are calculated to very high accuracy through a combination of periodic and finite-cluster electronic structure calculations. The Hartree-Fock contributions are taken from earlier work in which plane-wave calculations were corrected for pseudo-potential errors. Molecular electronic structure calculations on finite clusters are then used to compute the correlation contributions and finite-size effects are removed through the hierarchical scheme. The systematic improvability of the molecular electronic structure methods makes it possible to converge the static cohesive energy to within a few tenths of a millihartree. Zero-point energy contributions are determined from density functional theory phonon frequencies. All calculated properties of lithium hydride and deuteride agree with empirical observations to within experimental uncertainty
High-precision calculation of Hartree-Fock energy of crystals
No full textWhen using quantum chemistry techniques to calculate the energetics of bulk crystals, there is a need to calculate the Hartree-Fock (HF) energy of the crystal at the basis-set limit. We describe a strategy for achieving this, which exploits the fact that the HF energy of crystals can now be calculated using pseudopotentials and plane-wave basis sets, an approach that permits basis-set convergence to arbitary precision. The errors due to the use of pseudopotentials are then computed from the difference of all-electron and pseudopotential total energies of atomic clusters, extrapolated to the bulk-crystal limit. The strategy is tested for the case of the LiH crystal, and it is shown that the HF cohesive energy can be converged with respect to all technical parameters to a precision approaching 0.1 mE(h) per atom. This cohesive energy and the resulting HF value of the equilibrium lattice parameter are compared with literature values obtained using Gaussian basis sets
Energy benchmarks for water clusters and ice structures from an embedded many-body expansion.
No full textWe show how an embedded many-body expansion (EMBE) can be used to calculate accurate ab initio energies of water clusters and ice structures using wavefunction-based methods. We use the EMBE described recently by Bygrave et al. [J. Chem. Phys. 137, 164102 (2012)], in which the terms in the expansion are obtained from calculations on monomers, dimers, etc., acted on by an approximate representation of the embedding field due to all other molecules in the system, this field being a sum of Coulomb and exchange-repulsion fields. Our strategy is to separate the total energy of the system into Hartree-Fock and correlation parts, using the EMBE only for the correlation energy, with the Hartree-Fock energy calculated using standard molecular quantum chemistry for clusters and plane-wave methods for crystals. Our tests on a range of different water clusters up to the 16-mer show that for the second-order Møller-Plesset (MP2) method the EMBE truncated at 2-body level reproduces to better than 0.1 mE(h)/monomer the correlation energy from standard methods. The use of EMBE for computing coupled-cluster energies of clusters is also discussed. For the ice structures Ih, II, and VIII, we find that MP2 energies near the complete basis-set limit reproduce very well the experimental values of the absolute and relative binding energies, but that the use of coupled-cluster methods for many-body correlation (non-additive dispersion) is essential for a full description. Possible future applications of the EMBE approach are suggested
Cyberbase Gradignan (documents)
No full textDocuments et travaux portant sur le corpus “Cyberbase Gradignan
Cyberbase Gradignan
No full textLe corpus 'Cyberbase Gradignan' a été recueilli de juillet 2010 à juin 2012 dans le cadre de l'expérimentation Cyber-base®Justice mise en œuvre la Maison d'Arrêt de Gradignan et finalisée à l'accès à l'information, à l'apprentissage de l'informatique et à l'enseignement. Il est constitué d'enregistrements audiovisuels portant, d'une part, sur les activités dans l'espace informatique de la Maison d'Arrêt et, d'autre part, sur des entretiens avec les différents acteurs. L'ensemble du corpus a d'abord été segmenté et indexé (l'indexation, à la fois contextuelle et thématique, a été reportée dans les noms de fichiers). Une partie des séquences a ensuite été transcrite et annotée
Energy benchmarks for methane-water systems from quantum Monte Carlo and second-order Møller-Plesset calculations.
No full textThe quantum Monte Carlo (QMC) technique is used to generate accurate energy benchmarks for methane-water clusters containing a single methane monomer and up to 20 water monomers. The benchmarks for each type of cluster are computed for a set of geometries drawn from molecular dynamics simulations. The accuracy of QMC is expected to be comparable with that of coupled-cluster calculations, and this is confirmed by comparisons for the CH4-H2O dimer. The benchmarks are used to assess the accuracy of the second-order Møller-Plesset (MP2) approximation close to the complete basis-set limit. A recently developed embedded many-body technique is shown to give an efficient procedure for computing basis-set converged MP2 energies for the large clusters. It is found that MP2 values for the methane binding energies and the cohesive energies of the water clusters without methane are in close agreement with the QMC benchmarks, but the agreement is aided by partial cancelation between 2-body and beyond-2-body errors of MP2. The embedding approach allows MP2 to be applied without loss of accuracy to the methane hydrate crystal, and it is shown that the resulting methane binding energy and the cohesive energy of the water lattice agree almost exactly with recently reported QMC values
Accurate and systematically improvable density functional theory embedding for correlated wavefunctions
No full textWe will describe quantum embedding methods for performing accurate and scalable electronic structure theory calcns. in large mol. systems, with application to clusters, liqs., transition metal complexes, and chem. reactions.[1] "Exact non-additive kinetic potentials for embedded d. functional theory. Goodpaster JD, Ananth N, Manby FR, and Miller TF, JCP, 133, 084103 (2010).[2] "A simple, exact d.-functional-theory embedding scheme. Manby FR, Stella M, Goodpaster JD, and Miller TF, JCTC, 8, 2564 (2012).[3] "Accurate and systematically improvable d. functional theory embedding for correlated wavefunctions. Goodpaster JD, Barnes TA, Manby FR, and Miller TF, JCP, 140, 18A507 (2014)
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