176 research outputs found
Superfluidity and Quantum Melting of para-Hydrogen clusters
Structural and superfluid properties of para-Hydrogen clusters of size up to
N=40 molecules, are studied at low temperature (0.5 K < T < 4 K) by Path
Integral Monte Carlo simulations. The superfluid fraction displays an
interesting, non-monotonic behavior for 22 < N < 30. We interpret this
dependence in terms of variations with N of the cluster structure.
Superfluidity is observed at low T in clusters of as many as 27 molecules; in
the temperature range considered here, quantum melting is observed in some
clusters, which freeze at high temperature
Energy estimators for random series path-integral methods
We perform a thorough analysis on the choice of estimators for random series
path integral methods. In particular, we show that both the thermodynamic
(T-method) and the direct (H-method) energy estimators have finite variances
and are straightforward to implement. It is demonstrated that the agreement
between the T-method and the H-method estimators provides an important
consistency check on the quality of the path integral simulations. We
illustrate the behavior of the various estimators by computing the total,
kinetic, and potential energies of a molecular hydrogen cluster using three
different path integral techniques. Statistical tests are employed to validate
the sampling strategy adopted as well as to measure the performance of the
parallel random number generator utilized in the Monte Carlo simulation. Some
issues raised by previous simulations of the hydrogen cluster are clarified.Comment: 15 pages, 1 figure, 3 table
Heat capacity estimators for random series path-integral methods by finite-difference schemes
Previous heat capacity estimators used in path integral simulations either
have large variances that grow to infinity with the number of path variables or
require the evaluation of first and second order derivatives of the potential.
In the present paper, we show that the evaluation of the total energy by the
T-method estimator and of the heat capacity by the TT-method estimator can be
implemented by a finite difference scheme in a stable fashion. As such, the
variances of the resulting estimators are finite and the evaluation of the
estimators requires the potential function only. By comparison with the task of
computing the partition function, the evaluation of the estimators requires k +
1 times more calls to the potential, where k is the order of the difference
scheme employed. Quantum Monte Carlo simulations for the Ne_13 cluster
demonstrate that a second order central-difference scheme should suffice for
most applications.Comment: 11 pages, 4 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.
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
Upon the existence of short-time approximations of any polynomial order for the computation of density matrices by path integral methods
In this article, I provide significant mathematical evidence in support of
the existence of short-time approximations of any polynomial order for the
computation of density matrices of physical systems described by arbitrarily
smooth and bounded from below potentials. While for Theorem 2, which is
``experimental'', I only provide a ``physicist's'' proof, I believe the present
development is mathematically sound. As a verification, I explicitly construct
two short-time approximations to the density matrix having convergence orders 3
and 4, respectively. Furthermore, in the Appendix, I derive the convergence
constant for the trapezoidal Trotter path integral technique. The convergence
orders and constants are then verified by numerical simulations. While the two
short-time approximations constructed are of sure interest to physicists and
chemists involved in Monte Carlo path integral simulations, the present article
is also aimed at the mathematical community, who might find the results
interesting and worth exploring. I conclude the paper by discussing the
implications of the present findings with respect to the solvability of the
dynamical sign problem appearing in real-time Feynman path integral
simulations.Comment: 19 pages, 4 figures; the discrete short-time approximations are now
treated as independent from their continuous version; new examples of
discrete short-time approximations of order three and four are given; a new
appendix containing a short review on Brownian motion has been added; also,
some additional explanations are provided here and there; this is the last
version; to appear in Phys. Rev.
Analysis of path integrals at low temperature : Box formula, occupation time and ergodic approximation
We study the low temperature behaviour of path integrals for a simple
one-dimensional model. Starting from the Feynman-Kac formula, we derive a new
functional representation of the density matrix at finite temperature, in terms
of the occupation times of Brownian motions constrained to stay within boxes
with finite sizes. From that representation, we infer a kind of ergodic
approximation, which only involves double ordinary integrals. As shown by its
applications to different confining potentials, the ergodic approximation turns
out to be quite efficient, especially in the low-temperature regime where other
usual approximations fail
Three applications of path integrals: equilibrium and kinetic isotope effects, and the temperature dependence of the rate constant of the [1,5] sigmatropic hydrogen shift in (Z)-1,3-pentadiene
Recent experiments have confirmed the importance of nuclear quantum effects
even in large biomolecules at physiological temperature. Here we describe how
the path integral formalism can be used to describe rigorously the nuclear
quantum effects on equilibrium and kinetic properties of molecules.
Specifically, we explain how path integrals can be employed to evaluate the
equilibrium (EIE) and kinetic (KIE) isotope effects, and the temperature
dependence of the rate constant. The methodology is applied to the [1,5]
sigmatropic hydrogen shift in pentadiene. Both the KIE and the temperature
dependence of the rate constant confirm the importance of tunneling and other
nuclear quantum effects as well as of the anharmonicity of the potential energy
surface. Moreover, previous results on the KIE were improved by using a
combination of a high level electronic structure calculation within the
harmonic approximation with a path integral anharmonicity correction using a
lower level method.Comment: 9 pages, 4 figure
Caveolae-dependent and -independent uptake of albumin in cultured rodent pulmonary endothelial cells
Although a critical role for caveolae-mediated albumin transcytosis in pulmonary endothelium is well established, considerably less is known about caveolae-independent pathways. In this current study, we confirmed that cultured rat pulmonary microvascular (RPMEC) and pulmonary artery (RPAEC) endothelium endocytosed Alexa488-labeled albumin in a saturable, temperature-sensitive mode and internalization resulted in co-localization by fluorescence microscopy with cholera B toxin and caveolin-1. Although siRNA to caveolin-1 (cav-1) in RPAEC significantly inhibited albumin uptake, a remnant portion of albumin uptake was cav-1-independent, suggesting alternative pathways for albumin uptake. Thus, we isolated and cultured mouse lung endothelial cells (MLEC) from wild type and cav-1-/- mice and noted that ∼ 65% of albumin uptake, as determined by confocal imaging or live cell total internal reflectance fluorescence microscopy (TIRF), persisted in total absence of cav-1. Uptake of colloidal gold labeled albumin was evaluated by electron microscopy and demonstrated that albumin uptake in MLEC from cav-1-/- mice was through caveolae-independent pathway(s) including clathrin-coated pits that resulted in endosomal accumulation of albumin. Finally, we noted that albumin uptake in RPMEC was in part sensitive to pharmacological agents (amiloride [sodium transport inhibitor], Gö6976 [protein kinase C inhibitor], and cytochalasin D [inhibitor of actin polymerization]) consistent with a macropinocytosis-like process. The amiloride sensitivity accounting for macropinocytosis also exists in albumin uptake by both wild type and cav-1 -/- MLEC. We conclude from these studies that in addition to the well described caveolar-dependent pulmonary endothelial cell endocytosis of albumin, a portion of overall uptake in pulmonary endothelial cells is cav-1 insensitive and appears to involve clathrin-mediated endocytosis and macropinocytosis-like process. © 2013 Li et al
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