10,008 research outputs found
A parallel multistate framework for atomistic non-equilibrium reaction dynamics of solutes in strongly interacting organic solvents
We describe a parallel linear-scaling computational framework developed to
implement arbitrarily large multi-state empirical valence bond (MS-EVB)
calculations within CHARMM. Forces are obtained using the Hellman-Feynmann
relationship, giving continuous gradients, and excellent energy conservation.
Utilizing multi-dimensional Gaussian coupling elements fit to CCSD(T)-F12
electronic structure theory, we built a 64-state MS-EVB model designed to study
the F + CD3CN -> DF + CD2CN reaction in CD3CN solvent. This approach allows us
to build a reactive potential energy surface (PES) whose balanced accuracy and
efficiency considerably surpass what we could achieve otherwise. We use our PES
to run MD simulations, and examine a range of transient observables which
follow in the wake of reaction, including transient spectra of the DF
vibrational band, time dependent profiles of vibrationally excited DF in CD3CN
solvent, and relaxation rates for energy flow from DF into the solvent, all of
which agree well with experimental observations. Immediately following
deuterium abstraction, the nascent DF is in a non-equilibrium regime in two
different respects: (1) it is highly excited, with ~23 kcal mol-1 localized in
the stretch; and (2) not yet Hydrogen bonded to the CD3CN solvent, its
microsolvation environment is intermediate between the non-interacting
gas-phase limit and the solution-phase equilibrium limit. Vibrational
relaxation of the nascent DF results in a spectral blue shift, while relaxation
of its microsolvation environment results in a red shift. These two competing
effects result in a post-reaction relaxation profile distinct from that
observed when DF vibration excitation occurs within an equilibrium
microsolvation environment. The parallel software framework presented in this
paper should be more broadly applicable to a range of complex reactive systems.Comment: 58 pages and 29 Figure
Backstepping controller synthesis and characterizations of incremental stability
Incremental stability is a property of dynamical and control systems,
requiring the uniform asymptotic stability of every trajectory, rather than
that of an equilibrium point or a particular time-varying trajectory. Similarly
to stability, Lyapunov functions and contraction metrics play important roles
in the study of incremental stability. In this paper, we provide
characterizations and descriptions of incremental stability in terms of
existence of coordinate-invariant notions of incremental Lyapunov functions and
contraction metrics, respectively. Most design techniques providing controllers
rendering control systems incrementally stable have two main drawbacks: they
can only be applied to control systems in either parametric-strict-feedback or
strict-feedback form, and they require these control systems to be smooth. In
this paper, we propose a design technique that is applicable to larger classes
of (not necessarily smooth) control systems. Moreover, we propose a recursive
way of constructing contraction metrics (for smooth control systems) and
incremental Lyapunov functions which have been identified as a key tool
enabling the construction of finite abstractions of nonlinear control systems,
the approximation of stochastic hybrid systems, source-code model checking for
nonlinear dynamical systems and so on. The effectiveness of the proposed
results in this paper is illustrated by synthesizing a controller rendering a
non-smooth control system incrementally stable as well as constructing its
finite abstraction, using the computed incremental Lyapunov function.Comment: 23 pages, 2 figure
Combining Subgoal Graphs with Reinforcement Learning to Build a Rational Pathfinder
In this paper, we present a hierarchical path planning framework called SG-RL
(subgoal graphs-reinforcement learning), to plan rational paths for agents
maneuvering in continuous and uncertain environments. By "rational", we mean
(1) efficient path planning to eliminate first-move lags; (2) collision-free
and smooth for agents with kinematic constraints satisfied. SG-RL works in a
two-level manner. At the first level, SG-RL uses a geometric path-planning
method, i.e., Simple Subgoal Graphs (SSG), to efficiently find optimal abstract
paths, also called subgoal sequences. At the second level, SG-RL uses an RL
method, i.e., Least-Squares Policy Iteration (LSPI), to learn near-optimal
motion-planning policies which can generate kinematically feasible and
collision-free trajectories between adjacent subgoals. The first advantage of
the proposed method is that SSG can solve the limitations of sparse reward and
local minima trap for RL agents; thus, LSPI can be used to generate paths in
complex environments. The second advantage is that, when the environment
changes slightly (i.e., unexpected obstacles appearing), SG-RL does not need to
reconstruct subgoal graphs and replan subgoal sequences using SSG, since LSPI
can deal with uncertainties by exploiting its generalization ability to handle
changes in environments. Simulation experiments in representative scenarios
demonstrate that, compared with existing methods, SG-RL can work well on
large-scale maps with relatively low action-switching frequencies and shorter
path lengths, and SG-RL can deal with small changes in environments. We further
demonstrate that the design of reward functions and the types of training
environments are important factors for learning feasible policies.Comment: 20 page
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