3,166 research outputs found
An effective Hamiltonian approach for Donor-Bridge-Acceptor electronic transitions: Exploring the role of bath memory
We present here a formally exact model for electronic transitions between an
initial (donor) and final (acceptor) states linked by an intermediate (bridge)
state. Our model incorporates a common set of vibrational modes that are
coupled to the donor, bridge, and acceptor states and serves as a dissipative
bath that destroys quantum coherence between the donor and acceptor. Taking the
memory time of the bath as a free parameter, we calculate transition rates for
a heuristic 3-state/2 mode Hamiltonian system parameterized to represent the
energetics and couplings in a typical organic photovoltaic system. Our results
indicate that if the memory time of the bath is of the order of 10-100 fs, a
two-state kinetic (i.e., incoherent hopping) model will grossly underestimate
overall transition rate.Comment: 9 pages, 2 figure
Decoherent Histories and Non-adiabatic Quantum Molecular Dynamics
The role of quantum coherence loss in mixed quantum-classical dynamical
systems is explored in the context of the theory of quantum decoherence
introduced recently by Bittner and Rossky. (J. Chem. Phys. {\bf 103}, 8130
(1995)). This theory, which is based upon the consistent histories
interpretation of quantum mechanics, introduces decoherence in the quantum
subsystem by carefully considering the relevant time and length scales over
which one must consider the effects of phase interference between alternative
histories of the classical subsystem. Such alternative histories are an
integral part of any quantum-classical computational scheme which employ
transitions between discrete quantum states; consequently, the coherences
between alternative histories have a profound effect on the transition
probability between quantum states. In this paper, we review the Bittner-Rossky
theory and detail a computational algorithm suitable for large-scale quantum
molecular dynamics simulations which implements this theory. Application of the
algorithm towards the relaxation of a photoexcited aqueous electron compare
well to previous estimates of the excited state survival time as well as to the
experimental measurements.Comment: 22 pages, 3 figure
Exciton dissociation at donor-acceptor polymer heterojunctions: quantum nonadiabatic dynamics and effective-mode analysis
The quantum-dynamical mechanism of photoinduced subpicosecond exciton
dissociation and the concomitant formation of a charge-separated state at a
TFB:F8BT polymer heterojunction is elucidated. The analysis is based upon a
two-state vibronic coupling Hamiltonian including an explicit 24-mode
representation of a phonon bath comprising high-frequency (CC stretch) and
low-frequency (torsional) modes. The initial relaxation behavior is
characterized by coherent oscillations, along with the decay through an
extended nonadiabatic coupling region. This region is located in the vicinity
of a conical intersection hypersurface. A central ingredient of the analysis is
a novel effective mode representation, which highlights the role of the
low-frequency modes in the nonadiabatic dynamics. Quantum dynamical simulations
were carried out using the multiconfiguration time-dependent Hartree (MCTDH)
method
Quantum Mechanics with Trajectories: Quantum Trajectories and Adaptive Grids
Although the foundations of the hydrodynamical formulation of quantum
mechanics were laid over 50 years ago, it has only been within the past few
years that viable computational implementations have been developed. One
approach to solving the hydrodynamic equations uses quantum trajectories as the
computational tool. The trajectory equations of motion are described and
methods for implementation are discussed, including fitting of the fields to
gaussian clusters.Comment: Prepared for CiSE, Computing in Science and Engineering IEEE/AIP
special issue on computational chemistr
Soccer: is scoring goals a predictable Poissonian process?
The non-scientific event of a soccer match is analysed on a strictly
scientific level. The analysis is based on the recently introduced concept of a
team fitness (Eur. Phys. J. B 67, 445, 2009) and requires the use of
finite-size scaling. A uniquely defined function is derived which
quantitatively predicts the expected average outcome of a soccer match in terms
of the fitness of both teams. It is checked whether temporary fitness
fluctuations of a team hamper the predictability of a soccer match.
To a very good approximation scoring goals during a match can be
characterized as independent Poissonian processes with pre-determined
expectation values. Minor correlations give rise to an increase of the number
of draws. The non-Poissonian overall goal distribution is just a consequence of
the fitness distribution among different teams. The limits of predictability of
soccer matches are quantified. Our model-free classification of the underlying
ingredients determining the outcome of soccer matches can be generalized to
different types of sports events
Phonon-driven ultrafast exciton dissociation at donor-acceptor polymer heterojunctions
A quantum-dynamical analysis of phonon-driven exciton dissociation at polymer
heterojunctions is presented, using a hierarchical electron-phonon model
parameterized for three electronic states and 24 vibrational modes. Two
interfering decay pathways are identified: a direct charge separation, and an
indirect pathway via an intermediate bridge state. Both pathways depend
critically on the dynamical interplay of high-frequency C=C stretch modes and
low-frequency ring-torsional modes. The ultrafast, highly non-equilibrium
dynamics is consistent with time-resolved spectroscopic observations
Spins coupled to a -Regge lattice in 4d
We study an Ising spin system coupled to a fluctuating four-dimensional
-Regge lattice and compare with the results of the four-dimensional Ising
model on a regular lattice. Particular emphasis is placed on the phase
transition of the spin system and the associated critical exponents. We present
results from finite-size scaling analyses of extensive Monte Carlo simulations
which are consistent with mean-field predictions.Comment: Lattice2001(surfaces), 3 pages, 2 figure
Parallel-tempering cluster algorithm for computer simulations of critical phenomena
In finite-size scaling analyses of Monte Carlo simulations of second-order
phase transitions one often needs an extended temperature range around the
critical point. By combining the parallel tempering algorithm with cluster
updates and an adaptive routine to find the temperature window of interest, we
introduce a flexible and powerful method for systematic investigations of
critical phenomena. As a result, we gain one to two orders of magnitude in the
performance for 2D and 3D Ising models in comparison with the recently proposed
Wang-Landau recursion for cluster algorithms based on the multibondic
algorithm, which is already a great improvement over the standard
multicanonical variant.Comment: pages, 5 figures, and 2 table
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Maintenance and degradation of proteins in intact and severed axons: Implications for the mechanism of long-term survival of anucleate crayfish axons
Protein maintenance and degradation are examined in the
severed distal (anucleate) portions of crayfish medial giant
axons (MGAs), which remain viable for over 7 months following
axotomy. On polyacrylamide gels, the silver-stained
protein banding pattern of anucleate MGAs severed from
their cell bodies for up to 4 months remains remarkably similar
to that of intact MGAs. At 7 months postseverance, some
(but not all) proteins are decreased in anucleate MGAs compared
to intact MGAs. To determine the half-life of axonally
transported proteins, we radiolabeled MGA cell bodies and
monitored the degradation of newly synthesized transported
proteins. Assuming exponential decay, proteins in the fast
component of axonal transport have an average half-life of
14 d in anucleate MGAs and proteins in the slow component
have an average half-life of 17 d. Such half-lives are very
unlikely to account for the ability of anucleate MGAs to survive
for over 7 months after axotomy.This work was supported by an ATP grant to G.D.B.Neuroscienc
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