1,783,971 research outputs found
Transition state theory and the dynamics of hard disks
The dynamics of two and five disk systems confined in a square has been
studied using molecular dynamics simulations and compared with the predictions
of transition state theory. We determine the partition functions Z and
Z^\ddagger of transition state theory using a procedure first used by Salsburg
and Wood for the pressure. Our simulations show this procedure and transition
state theory are in excellent agreement with the simulations. A generalization
of the transition state theory to the case of a large number of disks N is made
and shown to be in full agreement with simulations of disks moving in a narrow
channel. The same procedure for hard spheres in three dimensions leads to the
Vogel-Fulcher-Tammann formula for their alpha relaxation time.Comment: 1 new author, new simulations and figures, less speculation. Now 6
pages, 6 figures, 1 animation. Animation may be viewed at
http://www.theory.physics.manchester.ac.uk/~godfrey/supplement/activated_dynamics2.htm
Femtosecond transition-state dynamics
This article presents the progress made in probing femtosecond transition–state dynamics of elementary reactions. Experiments demonstrating the dynamics in systems characterized by a transition region and by a saddle-point transition state are reported, and comparison with theory is made
Automated transition state theory calculations for high-throughput kinetics
A scarcity of known chemical kinetic parameters leads to the use of many
reaction rate estimates, which are not always sufficiently accurate, in the
construction of detailed kinetic models. To reduce the reliance on these
estimates and improve the accuracy of predictive kinetic models, we have
developed a high-throughput, fully automated, reaction rate calculation method,
AutoTST. The algorithm integrates automated saddle-point geometry search
methods and a canonical transition state theory kinetics calculator. The
automatically calculated reaction rates compare favorably to existing estimated
rates. Comparison against high level theoretical calculations show the new
automated method performs better than rate estimates when the estimate is made
by a poor analogy. The method will improve by accounting for internal rotor
contributions and by improving methods to determine molecular symmetry.Comment: 29 pages, 8 figure
The Transition State in a Noisy Environment
Transition State Theory overestimates reaction rates in solution because
conventional dividing surfaces between reagents and products are crossed many
times by the same reactive trajectory. We describe a recipe for constructing a
time-dependent dividing surface free of such recrossings in the presence of
noise. The no-recrossing limit of Transition State Theory thus becomes
generally available for the description of reactions in a fluctuating
environment
Chaotic Dynamics in Multidimensional Transition States
The crossing of a transition state in a multidimensional reactive system is
mediated by invariant geometric objects in phase space: An invariant
hyper-sphere that represents the transition state itself and invariant
hyper-cylinders that channel the system towards and away from the transition
state. The existence of these structures can only be guaranteed if the
invariant hyper-sphere is normally hyperbolic, i.e., the dynamics within the
transition state is not too strongly chaotic. We study the dynamics within the
transition state for the hydrogen exchange reaction in three degrees of
freedom. As the energy increases, the dynamics within the transition state
becomes increasingly chaotic. We find that the transition state first looses
and then, surprisingly, regains its normal hyperbolicity. The important phase
space structures of transition state theory will therefore exist at most
energies above the threshold
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