506 research outputs found
Adaptive Thermostats for Noisy Gradient Systems
We study numerical methods for sampling probability measures in high
dimension where the underlying model is only approximately identified with a
gradient system. Extended stochastic dynamical methods are discussed which have
application to multiscale models, nonequilibrium molecular dynamics, and
Bayesian sampling techniques arising in emerging machine learning applications.
In addition to providing a more comprehensive discussion of the foundations of
these methods, we propose a new numerical method for the adaptive
Langevin/stochastic gradient Nos\'{e}--Hoover thermostat that achieves a
dramatic improvement in numerical efficiency over the most popular stochastic
gradient methods reported in the literature. We also demonstrate that the newly
established method inherits a superconvergence property (fourth order
convergence to the invariant measure for configurational quantities) recently
demonstrated in the setting of Langevin dynamics. Our findings are verified by
numerical experiments
Hamiltonian ABC
Approximate Bayesian computation (ABC) is a powerful and elegant framework
for performing inference in simulation-based models. However, due to the
difficulty in scaling likelihood estimates, ABC remains useful for relatively
low-dimensional problems. We introduce Hamiltonian ABC (HABC), a set of
likelihood-free algorithms that apply recent advances in scaling Bayesian
learning using Hamiltonian Monte Carlo (HMC) and stochastic gradients. We find
that a small number forward simulations can effectively approximate the ABC
gradient, allowing Hamiltonian dynamics to efficiently traverse parameter
spaces. We also describe a new simple yet general approach of incorporating
random seeds into the state of the Markov chain, further reducing the random
walk behavior of HABC. We demonstrate HABC on several typical ABC problems, and
show that HABC samples comparably to regular Bayesian inference using true
gradients on a high-dimensional problem from machine learning.Comment: Submission to UAI 201
Least-biased correction of extended dynamical systems using observational data
We consider dynamical systems evolving near an equilibrium statistical state
where the interest is in modelling long term behavior that is consistent with
thermodynamic constraints. We adjust the distribution using an
entropy-optimizing formulation that can be computed on-the- fly, making
possible partial corrections using incomplete information, for example measured
data or data computed from a different model (or the same model at a different
scale). We employ a thermostatting technique to sample the target distribution
with the aim of capturing relavant statistical features while introducing mild
dynamical perturbation (thermostats). The method is tested for a point vortex
fluid model on the sphere, and we demonstrate both convergence of equilibrium
quantities and the ability of the formulation to balance stationary and
transient- regime errors.Comment: 27 page
High-Order Stochastic Gradient Thermostats for Bayesian Learning of Deep Models
Learning in deep models using Bayesian methods has generated significant
attention recently. This is largely because of the feasibility of modern
Bayesian methods to yield scalable learning and inference, while maintaining a
measure of uncertainty in the model parameters. Stochastic gradient MCMC
algorithms (SG-MCMC) are a family of diffusion-based sampling methods for
large-scale Bayesian learning. In SG-MCMC, multivariate stochastic gradient
thermostats (mSGNHT) augment each parameter of interest, with a momentum and a
thermostat variable to maintain stationary distributions as target posterior
distributions. As the number of variables in a continuous-time diffusion
increases, its numerical approximation error becomes a practical bottleneck, so
better use of a numerical integrator is desirable. To this end, we propose use
of an efficient symmetric splitting integrator in mSGNHT, instead of the
traditional Euler integrator. We demonstrate that the proposed scheme is more
accurate, robust, and converges faster. These properties are demonstrated to be
desirable in Bayesian deep learning. Extensive experiments on two canonical
models and their deep extensions demonstrate that the proposed scheme improves
general Bayesian posterior sampling, particularly for deep models.Comment: AAAI 201
On the effect of the thermostat in non-equilibrium molecular dynamics simulations
The numerical investigation of the statics and dynamics of systems in
nonequilibrium in general, and under shear flow in particular, has become more
and more common. However, not all the numerical methods developed to simulate
equilibrium systems can be successfully adapted to out-of-equilibrium cases.
This is especially true for thermostats. Indeed, even though thermostats
developed to work under equilibrium conditions sometimes display good agreement
with rheology experiments, their performance rapidly degrades beyond weak
dissipation and small shear rates. Here we focus on gauging the relative
performances of three thermostats, Langevin, dissipative particle dynamics, and
Bussi-Donadio-Parrinello under varying parameters and external conditions. We
compare their effectiveness by looking at different observables and clearly
demonstrate that choosing the right thermostat (and its parameters) requires a
careful evaluation of, at least, temperature, density and velocity profiles. We
also show that small modifications of the Langevin and DPD thermostats greatly
enhance their performance in a wide range of parameters.Comment: 13 pages, 9 figure
- …