15,140 research outputs found
Theoretical study of the finite temperature spectroscopy in van der Waals clusters. III Solvated Chromophore as an effective diatomics
The absorption spectroscopy of calcium-doped argon clusters is described in
terms of an effective diatomics molecule Ca-(Ar_n), in the framework of
semiclassical vertical transitions. We show how, upon choosing a suitable
reaction coordinate, the effective finite-temperature equilibrium properties
can be obtained for the ground- and excited-surfaces from the potential of mean
force (PMF). An extension of the recent multiple range random-walk method is
used to calculate the PMF over continuous intervals of distances. The
absorption spectra calculated using this single-coordinate description are
found to be in good agreement with the spectra obtained from high-statistics
Monte Carlo data, in various situations. For CaAr, we compare the
performances of two different choices of the reaction coordinate. For CaAr_37,
the method is seen to be accurate enough to distinguish between different
low-energy structures. Finally, the idea of casting the initial many-body
problem into a single degree of freedom problem is tested on the spectroscopy
of calcium in bulk solid argon.Comment: 8 pages, 9 figure
Theoretical study of finite temperature spectroscopy in van der Waals clusters. II Time-dependent absorption spectra
Using approximate partition functions and a master equation approach, we
investigate the statistical relaxation toward equilibrium in selected CaAr
clusters. The Gaussian theory of absorption (previous article) is employed to
calculate the average photoabsorption intensity associated with the 4s^2->
4s^14p^1 transition of calcium as a function of time during relaxation. In
CaAr_6 and CaAr_10 simple relaxation is observed with a single time scale.
CaAr_13 exhibits much slower dynamics and the relaxation occurs over two
distinct time scales. CaAr_37 shows much slower relaxation with multiple
transients, reminiscent of glassy behavior due to competition between different
low-energy structures. We interpret these results in terms of the underlying
potential energy surfaces for these clusters.Comment: 10 pages, 9 figure
Stacked clusters of polycyclic aromatic hydrocarbon molecules
Clusters of polycyclic aromatic hydrocarbon (PAH) molecules are modelled
using explicit all-atom potentials using a rigid body approximation. The PAH's
considered range from pyrene (C10H8) to circumcoronene (C54H18), and clusters
containing between 2 and 32 molecules are investigated. In addition to the
usual repulsion-dispersion interactions, electrostatic point-charge
interactions are incorporated, as obtained from density functional theory
calculations. The general electrostatic distribution in neutral or singly
charged PAH's is reproduced well using a fluctuating charges analysis, which
provides an adequate description of the multipolar distribution. Global
optimization is performed using a variety of methods, including basin-hopping
and parallel tempering Monte Carlo. We find evidence that stacking the PAH
molecules generally yields the most stable motif. A structural transition
between one-dimensional stacks and three-dimensional shapes built from mutiple
stacks is observed at larger sizes, and the threshold for this transition
increases with the size of the monomer. Larger aggregates seem to evolve toward
the packing observed for benzene in bulk.Difficulties met in optimizing these
clusters are analysed in terms of the strong anisotropy of the molecules. We
also discuss segregation in heterogeneous clusters and vibrational properties
in the context of astrophysical observations.Comment: 12 pages, 7 figure
Composition-induced structural transitions in mixed rare-gas clusters
The low-energy structures of mixed Ar--Xe and Kr--Xe Lennard-Jones clusters
are investigated using a newly developed parallel Monte Carlo minimization
algorithm with specific exchange moves between particles or trajectories. Tests
on the 13- and 19- atom clusters show a significant improvement over the
conventional basin-hopping method, the average search length being reduced by
more than one order of magnitude. The method is applied to the more difficult
case of the 38-atom cluster, for which the homogeneous clusters have a
truncated octahedral shape. It is found that alloys of dissimilar elements
(Ar--Xe) favor polytetrahedral geometries over octahedra due to the reduced
strain penalty. Conversely, octahedra are even more stable in Kr--Xe alloys
than in Kr_38 or Xe_38, and they show a core-surface phase separation behavior.
These trends are indeed also observed and further analysed on the 55-atom
cluster. Finally, we correlate the relative stability of cubic structures in
these clusters to the glassforming character of the bulk mixtures.Comment: 14 pages, 8 figures, 5 tables PRB vol 70, in pres
Phase changes in 38 atom Lennard-Jones clusters. II: A parallel tempering study of equilibrium and dynamic properties in the molecular dynamics and microcanonical
We study the 38-atom Lennard-Jones cluster with parallel tempering Monte
Carlo methods in the microcanonical and molecular dynamics ensembles. A new
Monte Carlo algorithm is presented that samples rigorously the molecular
dynamics ensemble for a system at constant total energy, linear and angular
momenta. By combining the parallel tempering technique with molecular dynamics
methods, we develop a hybrid method to overcome quasi-ergodicity and to extract
both equilibrium and dynamical properties from Monte Carlo and molecular
dynamics simulations. Several thermodynamic, structural and dynamical
properties are investigated for LJ, including the caloric curve, the
diffusion constant and the largest Lyapunov exponent. The importance of
insuring ergodicity in molecular dynamics simulations is illustrated by
comparing the results of ergodic simulations with earlier molecular dynamics
simulations.Comment: Journal of Chemical Physics, accepte
Evaluation of a microbial muramidase supplementation on growth performance, apparent ileal digestibility, and intestinal histology of broiler chickens
The current study evaluated the effects of different inclusion levels of microbial muramidase (Muramidase 007, DSM Nutritional Products) on gastrointestinal functionality by determination of apparent ileal digestibility (AID) of nutrients, investigation of intestinal histomorphology, and quantification of resulting growth performance. Four maize-wheat-soybean experimental diets were produced without (C) and with different dosages of muramidase: low (L, 25,000 LSU(F)/kg), medium (M, 35,000 LSU(F)/kg), and high (H, 45,000 LSU(F)/kg); diets were fed to broilers for 35 d. At the end of the experiment, AID of ether extract (EE), crude protein (CP), Ca, and P were determined and samples of the mid-jejunum and -ileum were collected for histomorphological observations. Data were subjected to ANOVA analysis using the GLM procedure. Orthogonal polynomial contrasts were used to assess linear and quadratic effects of different levels of the muramidase. At the end of the trial, Muramidase 007 supplementation linearly increased body weight gain and decreased feed conversion ratio (FCR) (P †0.05). Adding the muramidase to broiler diets also linearly increased the European poultry efficiency factor (P †0.05). Inclusion of the muramidase in broiler diets linearly increased AID of CP, EE, and P (P †0.05), and the H group had a higher AID of EE and CP compared to C group (P †0.05). Microbial muramidase supplementation linearly increased ileal villus length to crypt depth ratio and decreased the number of ileal CD45 cells (P †0.05). Broilers fed M and H diets had fewer number of CD45 cells in the ileum compared to those in C group (P †0.05). In conclusion, the results of the present study demonstrated that inclusion of the microbial muramidase in broiler diets could increase AID of key nutrients and improve growth performance in broilers. Adding microbial muramidase to broiler diets can therefore be considered as an interesting prospect to improve gastrointestinal functionality. Biological mechanisms causing these improvements need to be studied further
Theoretical study of finite temperature spectroscopy in van der Waals clusters. I. Probing phase changes in CaAr_n
The photoabsorption spectra of calcium-doped argon clusters CaAr_n are
investigated at thermal equilibrium using a variety of theoretical and
numerical tools. The influence of temperature on the absorption spectra is
estimated using the quantum superposition method for a variety of cluster sizes
in the range 6<=n<=146. At the harmonic level of approximation, the absorption
intensity is calculated through an extension of the Gaussian theory by Wadi and
Pollak [J. Chem. Phys. vol 110, 11890 (1999)]. This theory is tested on simple,
few-atom systems in both the classical and quantum regimes for which highly
accurate Monte Carlo data can be obtained. By incorporating quantum anharmonic
corrections to the partition functions and respective weights of the isomers,
we show that the superposition method can correctly describe the
finite-temperature spectroscopic properties of CaAr_n systems. The use of the
absorption spectrum as a possible probe of isomerization or phase changes in
the argon cluster is discussed at the light of finite-size effects.Comment: 17 pages, 9 figure
Dual branes in topological sigma models over Lie groups. BF-theory and non-factorizable Lie bialgebras
We complete the study of the Poisson-Sigma model over Poisson-Lie groups.
Firstly, we solve the models with targets and (the dual group of the
Poisson-Lie group ) corresponding to a triangular -matrix and show that
the model over is always equivalent to BF-theory. Then, given an
arbitrary -matrix, we address the problem of finding D-branes preserving the
duality between the models. We identify a broad class of dual branes which are
subgroups of and , but not necessarily Poisson-Lie subgroups. In
particular, they are not coisotropic submanifolds in the general case and what
is more, we show that by means of duality transformations one can go from
coisotropic to non-coisotropic branes. This fact makes clear that
non-coisotropic branes are natural boundary conditions for the Poisson-Sigma
model.Comment: 24 pages; JHEP style; Final versio
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