229 research outputs found
Thermodynamics and equilibrium structure of Ne_38 cluster: Quantum Mechanics versus Classical
The equilibrium properties of classical LJ_38 versus quantum Ne_38
Lennard-Jones clusters are investigated. The quantum simulations use both the
Path-Integral Monte-Carlo (PIMC) and the recently developed
Variational-Gaussian-Wavepacket Monte-Carlo (VGW-MC) methods. The PIMC and the
classical MC simulations are implemented in the parallel tempering framework.
The VGW method is used to locate and characterize the low energy states of
Ne_38, which are then further refined by PIMC calculations. Unlike the
classical case, the ground state of Ne_38 is a liquid-like structure. Among the
several liquid-like states with energies below the two symmetric states (O_h
and C_5v), the lowest two exhibit strong delocalization over basins associated
with at least two classical local minima. Because the symmetric structures do
not play an essential role in the thermodynamics of Ne_38, the quantum heat
capacity is a featureless curve indicative of the absence of any structural
transformations. Good agreement between the two methods, VGW and PIMC, is
obtained.Comment: 13 pages, 9 figure
Reconstruction of thermally-symmetrized quantum autocorrelation functions from imaginary-time data
In this paper, I propose a technique for recovering quantum dynamical
information from imaginary-time data via the resolution of a one-dimensional
Hamburger moment problem. It is shown that the quantum autocorrelation
functions are uniquely determined by and can be reconstructed from their
sequence of derivatives at origin. A general class of reconstruction algorithms
is then identified, according to Theorem 3. The technique is advocated as
especially effective for a certain class of quantum problems in continuum
space, for which only a few moments are necessary. For such problems, it is
argued that the derivatives at origin can be evaluated by Monte Carlo
simulations via estimators of finite variances in the limit of an infinite
number of path variables. Finally, a maximum entropy inversion algorithm for
the Hamburger moment problem is utilized to compute the quantum rate of
reaction for a one-dimensional symmetric Eckart barrier.Comment: 15 pages, no figures, to appear in Phys. Rev.
Impaired Caveolae Function and Upregulation of Alternative Endocytic Pathways Induced by Experimental Modulation of Intersectin-1s Expression in Mouse Lung Endothelium
Intersectin-1s (ITSN-1s), a protein containing five SH3 (A-E) domains, regulates via the SH3A the function of dynamin-2 (dyn2) at the endocytic site. ITSN-1s expression was modulated in mouse lung endothelium by liposome delivery of either a plasmid cDNA encoding myc-SH3A or a specific siRNA targeting ITSN-1 gene. The lung vasculature of SH3A-transduced and ITSN-1s- deficient mice was perfused with gold albumin (Au-BSA) to analyze by electron microscopy the morphological intermediates and pathways involved in transendothelial transport or with dinitrophenylated (DNP)-BSA to quantify by ELISA its transport. Acute modulation of ITSN-1s expression decreased the number of caveolae, impaired their transport, and opened the interendothelial junctions, while upregulating compensatory nonconventional endocytic/transcytotic structures. Chronic inhibition of ITSN-1s further increased the occurrence of nonconventional intermediates and partially restored the junctional integrity. These findings indicate that ITSN-1s expression is required for caveolae function and efficient transendothelial transport. Moreover, our results demonstrate that ECs are highly adapted to perform their transport function while maintaining lung homeostasis
Extrapolated High-Order Propagators for Path Integral Monte Carlo Simulations
We present a new class of high-order imaginary time propagators for
path-integral Monte Carlo simulations by subtracting lower order propagators.
By requiring all terms of the extrapolated propagator be sampled uniformly, the
subtraction only affects the potential part of the path integral. The
negligible violation of positivity of the resulting path integral at small time
steps has no discernable affect on the accuracy of our method. Thus in
principle arbitrarily high order algorithms can be devised for path-integral
Monte Carlo simulations. We verify this claim is by showing that fourth, sixth,
and eighth order convergence can indeed be achieved in solving for the ground
state of strongly interacting quantum many-body systems such as bulk liquid
He.Comment: 9 pages and 3 figures. Submitted to J. Chem. Phy
Entropic effects in large-scale Monte Carlo simulations
The efficiency of Monte Carlo samplers is dictated not only by energetic
effects, such as large barriers, but also by entropic effects that are due to
the sheer volume that is sampled. The latter effects appear in the form of an
entropic mismatch or divergence between the direct and reverse trial moves. We
provide lower and upper bounds for the average acceptance probability in terms
of the Renyi divergence of order 1/2. We show that the asymptotic finitude of
the entropic divergence is the necessary and sufficient condition for
non-vanishing acceptance probabilities in the limit of large dimensions.
Furthermore, we demonstrate that the upper bound is reasonably tight by showing
that the exponent is asymptotically exact for systems made up of a large number
of independent and identically distributed subsystems. For the last statement,
we provide an alternative proof that relies on the reformulation of the
acceptance probability as a large deviation problem. The reformulation also
leads to a class of low-variance estimators for strongly asymmetric
distributions. We show that the entropy divergence causes a decay in the
average displacements with the number of dimensions n that are simultaneously
updated. For systems that have a well-defined thermodynamic limit, the decay is
demonstrated to be n^{-1/2} for random-walk Monte Carlo and n^{-1/6} for Smart
Monte Carlo (SMC). Numerical simulations of the LJ_38 cluster show that SMC is
virtually as efficient as the Markov chain implementation of the Gibbs sampler,
which is normally utilized for Lennard-Jones clusters. An application of the
entropic inequalities to the parallel tempering method demonstrates that the
number of replicas increases as the square root of the heat capacity of the
system.Comment: minor corrections; the best compromise for the value of the epsilon
parameter in Eq. A9 is now shown to be log(2); 13 pages, 4 figures, to appear
in PR
The Public Pension System in Romania – Analysis Before and After 1999
Objectives This paper aims to examine the public pension system in Romania and the significant differences that 1999 brought it. It is important to see if system effects such as the number of beneficiaries and collapse on the number of taxpayers are experienced in present. It explores the main types of pension granted in the public pension system - pensions for old age. Approach It is an attempt to identify the main sources (contributions owed by employers and employees) and also the way pensions are calculated and given before and after 1999. Results We conclude that public pension system in Romania has suffered many changes in a positive way. Implications For taxpayers, both employers and employees it is important that the public pension system work in optimal conditions, that the minimum and maximum contribution "stage" take the proper values. Value Knowing the importance of the public pension system in the Romanian society will know what measures should be taken to improve it
Phase changes in selected Lennard-Jones X_{13-n}Y_n clusters
Detailed studies of the thermodynamic properties of selected binary
Lennard-Jones clusters of the type X_{13-n}Y_n (where n=1,2,3) are presented.
The total energy, heat capacity and first derivative of the heat capacity as a
function of temperature are calculated by using the classical and path integral
Monte Carlo methods combined with the parallel tempering technique. A
modification in the phase change phenomena from the presence of impurity atoms
and quantum effects is investigated.Comment: 14 pages, 13 figures. submitted to J. Chem. Phy
Fast Convergence of Path Integrals for Many-body Systems
We generalize a recently developed method for accelerated Monte Carlo
calculation of path integrals to the physically relevant case of generic
many-body systems. This is done by developing an analytic procedure for
constructing a hierarchy of effective actions leading to improvements in
convergence of -fold discretized many-body path integral expressions from
1/N to for generic . In this paper we present explicit solutions
within this hierarchy up to level . Using this we calculate the low lying
energy levels of a two particle model with quartic interactions for several
values of coupling and demonstrate agreement with analytical results governing
the increase in efficiency of the new method. The applicability of the
developed scheme is further extended to the calculation of energy expectation
values through the construction of associated energy estimators exhibiting the
same speedup in convergence.Comment: 20 pages, 6 figures, 1 tabl
Pro-inflammatory endothelial cell dysfunction is associated with intersectin-1s down-regulation
<p>Abstract</p> <p>Background</p> <p>The response of lung microvascular endothelial cells (ECs) to lipopolysaccharide (LPS) is central to the pathogenesis of lung injury. It is dual in nature, with one facet that is pro-inflammatory and another that is cyto-protective. In previous work, overexpression of the anti-apoptotic Bcl-X<sub>L</sub> rescued ECs from apoptosis triggered by siRNA knockdown of intersectin-1s (ITSN-1s), a pro-survival protein crucial for ECs function. Here we further characterized the cyto-protective EC response to LPS and pro-inflammatory dysfunction.</p> <p>Methods and Results</p> <p>Electron microscopy (EM) analyses of LPS-exposed ECs revealed an activated/dysfunctional phenotype, while a biotin assay for caveolae internalization followed by biochemical quantification indicated that LPS causes a 40% inhibition in biotin uptake compared to controls. Quantitative PCR and Western blotting were used to evaluate the mRNA and protein expression, respectively, for several regulatory proteins of intrinsic apoptosis, including ITSN-1s. The decrease in ITSN-1s mRNA and protein expression were countered by Bcl-X<sub>L</sub> and survivin upregulation, as well as Bim downregulation, events thought to protect ECs from impending apoptosis. Absence of apoptosis was confirmed by TUNEL and lack of cytochrome c (cyt c) efflux from mitochondria. Moreover, LPS exposure caused induction and activation of inducible nitric oxide synthase (iNOS) and a mitochondrial variant (mtNOS), as well as augmented mitochondrial NO production as measured by an oxidation oxyhemoglobin (oxyHb) assay applied on mitochondrial-enriched fractions prepared from LPS-exposed ECs. Interestingly, expression of myc-ITSN-1s rescued caveolae endocytosis and reversed induction of iNOS expression.</p> <p>Conclusion</p> <p>Our results suggest that ITSN-1s deficiency is relevant for the pro-inflammatory ECs dysfunction induced by LPS.</p
- …