124 research outputs found
Optimized Jastrow-Slater wave functions for ground and excited states: Application to the lowest states of ethene
A quantum Monte Carlo method is presented for determining multi-determinantal
Jastrow-Slater wave functions for which the energy is stationary with respect
to the simultaneous optimization of orbitals and configuration interaction
coefficients. The approach is within the framework of the so-called energy
fluctuation potential method which minimizes the energy in an iterative fashion
based on Monte Carlo sampling and a fitting of the local energy fluctuations.
The optimization of the orbitals is combined with the optimization of the
configuration interaction coefficients through the use of additional single
excitations to a set of external orbitals. A new set of orbitals is then
obtained from the natural orbitals of this enlarged configuration interaction
expansion. For excited states, the approach is extended to treat the average of
several states within the same irreducible representation of the pointgroup of
the molecule. The relationship of our optimization method with the stochastic
reconfiguration technique by Sorella et al. is examined. Finally, the
performance of our approach is illustrated with the lowest states of ethene, in
particular with the difficult case of the singlet 1B_1u state.Comment: 12 pages, 2 figure
Excitations in photoactive molecules from quantum Monte Carlo
Despite significant advances in electronic structure methods for the
treatment of excited states, attaining an accurate description of the
photoinduced processes in photoactive biomolecules is proving very difficult.
For the prototypical photosensitive molecules, formaldimine, formaldehyde and a
minimal protonated Schiff base model of the retinal chromophore, we investigate
the performance of various approaches generally considered promising for the
computation of excited potential energy surfaces. We show that quantum Monte
Carlo can accurately estimate the excitation energies of the studied systems if
one constructs carefully the trial wave function, including in most cases the
reoptimization of its determinantal part within quantum Monte Carlo. While
time-dependent density functional theory and quantum Monte Carlo are generally
in reasonable agreement, they yield a qualitatively different description of
the isomerization of the Schiff base model. Finally, we find that the
restricted open shell Kohn-Sham method is at variance with quantum Monte Carlo
in estimating the lowest-singlet excited state potential energy surface for
low-symmetry molecular structures.Comment: 10 pages, 6 figure
The creative use of drama in the curriculum of Christian education.
Thesis (M.A.)--Boston Universit
Contes et conteurs de Louqsor
Pour le professeur Werner VycichI « Raconte une belle histoire ou je te tue... » A. Khatibi, Ombres japonaises, Fata Morgana, 1988. Cette étude s'articule autour de trois axes. En premier lieu, je me propose d’« aller voir » du côté des conteurs égyptiens en quoi ils restent, aujourd'hui encore, les dépositaires d'une parole qui constitue la mémoire orale. J'analyse également le contexte et les règles selon lesquelles ils pratiquent leur art. . J'ai choisi la région de Louqsor parce qu'elle e..
Introduction to the variational and diffusion Monte Carlo methods
We provide a pedagogical introduction to the two main variants of real-space
quantum Monte Carlo methods for electronic-structure calculations: variational
Monte Carlo (VMC) and diffusion Monte Carlo (DMC). Assuming no prior knowledge
on the subject, we review in depth the Metropolis-Hastings algorithm used in
VMC for sampling the square of an approximate wave function, discussing details
important for applications to electronic systems. We also review in detail the
more sophisticated DMC algorithm within the fixed-node approximation,
introduced to avoid the infamous Fermionic sign problem, which allows one to
sample a more accurate approximation to the ground-state wave function.
Throughout this review, we discuss the statistical methods used for evaluating
expectation values and statistical uncertainties. In particular, we show how to
estimate nonlinear functions of expectation values and their statistical
uncertainties.Comment: Advances in Quantum Chemistry, 2015, Electron Correlation in
Molecules -- ab initio Beyond Gaussian Quantum Chemistry, pp.000
Quasi-classical Molecular Dynamics Simulations of the Electron Gas: Dynamic properties
Results of quasi-classical molecular dynamics simulations of the quantum
electron gas are reported. Quantum effects corresponding to the Pauli and the
Heisenberg principle are modeled by an effective momentum-dependent
Hamiltonian. The velocity autocorrelation functions and the dynamic structure
factors have been computed. A comparison with theoretical predictions was
performed.Comment: 8 figure
Segregation by thermal diffusion of an intruder in a moderately dense granular fluid
A solution of the inelastic Enskog equation that goes beyond the weak
dissipation limit and applies for moderate densities is used to determine the
thermal diffusion factor of an intruder immersed in a dense granular gas under
gravity. This factor provides a segregation criterion that shows the transition
between the Brazil-nut effect (BNE) and the reverse Brazil-nut effect (RBNE) by
varying the parameters of the system (masses, sizes, density and coefficients
of restitution). The form of the phase-diagrams for the BNE/RBNE transition
depends sensitively on the value of gravity relative to the thermal gradient,
so that it is possible to switch between both states for given values of the
parameters of the system. Two specific limits are considered with detail: (i)
absence of gravity, and (ii) homogeneous temperature. In the latter case, after
some approximations, our results are consistent with previous theoretical
results derived from the Enskog equation. Our results also indicate that the
influence of dissipation on thermal diffusion is more important in the absence
of gravity than in the opposite limit. The present analysis extends previous
theoretical results derived in the dilute limit case [V. Garz\'o, Europhys.
Lett. {\bf 75}, 521 (2006)] and is consistent with the findings of some recent
experimental results.Comment: 10 figure
Regenerative Energy Storage System for Space Exploration Missions
This paper describes the development and testing of a 1 kW reversible solid oxide fuel cell intended for energy storage on space exploration missions, particularly for long term Mars exploration. The energy is stored as H2 or CO produced by electrolysis of H2O or CO2. The reactants are then converted back to its original composition by producing electricity.
The breadboard was operated for 1250 hours alternating between electrolyser mode and fuel cell mode with H2/H2O as reactants. During the tests, as long as the mechanical integrity of the system was maintained, no degradation effect was observed. At the end of the test period, the fuel cell was operated for three full cycles (approx. 50 hours) with CO/CO2 as reactants. The performance on CO/CO2 was lower than for hydrogen, but sufficient to be used in a compact energy storage system for Mars exploration
Optimization of inhomogeneous electron correlation factors in periodic solids
A method is presented for the optimization of one-body and inhomogeneous
two-body terms in correlated electronic wave functions of Jastrow-Slater type.
The most general form of inhomogeneous correlation term which is compatible
with crystal symmetry is used and the energy is minimized with respect to all
parameters using a rapidly convergent iterative approach, based on Monte Carlo
sampling of the energy and fitting energy fluctuations. The energy minimization
is performed exactly within statistical sampling error for the energy
derivatives and the resulting one- and two-body terms of the wave function are
found to be well-determined. The largest calculations performed require the
optimization of over 3000 parameters. The inhomogeneous two-electron
correlation terms are calculated for diamond and rhombohedral graphite. The
optimal terms in diamond are found to be approximately homogeneous and
isotropic over all ranges of electron separation, but exhibit some
inhomogeneity at short- and intermediate-range, whereas those in graphite are
found to be homogeneous at short-range, but inhomogeneous and anisotropic at
intermediate- and long-range electron separation.Comment: 23 pages, 15 figures, 1 table, REVTeX4, submitted to PR
O Efeito da Complexidade Estrutural da Fonte de Carbono e Nitrogênio no Desempenho Fermentativo de Leveduras iIdustrias
Saccharomyces cerevisiae tem evoluído para utilizar melhor os nutrientes acessíveis e se adaptar as deficiências nutricionais de forma a maximizar sua sobrevivência. O presente trabalho avaliou o efeito das fontes de carbono e nitrogênio no desempenho fermentativo de leveduras industriais (Catanduva-1 e Pedra-2). As análises dos parâmetros fermentativos mostraram que o tipo da fonte de carbono e a complexidade estrutural da fonte nitrogenada causam efeitos diferentes sobre o metabolismo das linhagens industriais. As fontes de carbono contendo glicose e sacarose suplementadas com a fonte nitrogenada (peptona) apresentaram melhor desempenho fermentativo para as linhagens estudadas. Este estudo mostrou que a complexidade estrutural da fonte de nitrogênio, em correlação com o tipo de açúcar, tem um efeito importante no desempenho fermentativo de leveduras industriais
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