This is the seventh session of the second season of the Mixed-Gen on-line series aimed mainly at PhD students and researchers in their first post-doc. Our goal is to continue providing a new venue for these young scientists to share their work, get expert feedback and have an opportunity to strengthen scientific relations within the CECAM community.

The general area for this session is Simulating non-equilibrium phenomena and rare events

To participate

If you are a PhD student or a post-doc:

Please use the Participate Tab on this page to start the application. You will have to login using your CECAM account to access the application form. If you don't have a CECAM account yet, use the register option on the top right corner of the login page...and welcome to CECAM!

If you are a more senior scientist:

Please contact the organisers and we shall process your registration.

Submission of posters

(Please note that - at least for the time being - we shall accept posters only from PhD students or researchers in their first post-doc)

After your application is accepted, you will be able to submit a poster. In the CECAM page for this event, go to “My participation” tab and click on “Add a poster”, providing title and abstract following the recommended format. On the same form you can upload your poster file in png or jpg as soon as it is ready. These formats are strict to enable showing of the poster in the Gather session. If the poster file is not ready at the moment of submitting your abstract, you can upload it later by editing your submission (Go to “My participation” tab and click three vertical dots on “Actions” column on table “My posters”). Please upload your poster as soon as possible to enable a decision from the selection committee - see below.

Please note that posters will be visible in the Gather room associated with this session until the end of the series (June 2022) unless otherwise requested.

DEADLINE FOR SUBMISSION: TEN DAYS BEFORE THE EVENT

Selection of posters

Posters will be selected by the event organisers with the support of our main speaker and experts who will take place in the poster session. 

Selection of the two talks by PhD or first year postdocs

These contributions, to be broadcasted in the Zoom webinar in the first part of the event, will be selected, after a preliminary screening by the organisers, the main speaker and guest experts, via a lottery from the posters selected for the Gather session. Please tick “No” to the question “Upgrade to talk?” in your application if you DO NOT WANT your poster to be considered for this lottery. 

THE DECISION ON THE POSTER AND THE OUTCOME OF THE LOTTERY SELECTION WILL BE COMMUNICATED ONE WEEK BEFORE THE EVENT

POSTER SUBMISSIONS BEYOND THIS DEADLINE WILL BE ACCEPTED BUT NOT CONSIDERED FOR UPGRADE TO TALK. SUBMISSION WILL BE DEFINITELY CLOSED FOUR DAYS BEFORE THE EVENT.

SESSION 7. Title and abstract of talks

How to build a theory for a coarse-grained system out of equilibrium

Tanja Schilling, University of Freiburg

Active matter, responsive (“smart”) materials and materials under time-dependent load are systems out of thermal equilibrium. To construct coarse-grained models for such systems, one needs to integrate out a distribution of microstates that evolves in time. This is a challenging task. In this lecture, I will recall equilibrium coarse-graining methods, in particular projection operator formalisms. Then I will review recent developments in theoretical approaches to the non-equilibrium coarse-graining problem, in particular, time-dependent projection operator formalisms and numerical schemes to construct explicitly time-dependent memory kernels.

PDF presentation

Unbiased sampling of HMC schemes for non separable Hamiltonian systems

Régis Santet, Ecole des Ponts

Hamiltonian Monte Carlo (or Hybrid Monte Carlo) [1] is a Markov Chain Monte Carlo method that allows to sample high dimensional probability measures. Girolami et al.[2] have used it to include a position-dependent diffusion coefficient, coming from the overdamped Langevin dynamics, that improves the convergence of the numerical method. It however requires simulating Hamiltonian dynamics with a non separable Hamiltonian, which is done in practice with implicit methods in order to ensure the preservation of key properties of the Hamiltonian dynamics (symplecticity and time-reversibility in particular) [3]. Unfortunately, actual implicit numerical schemes cannot be reversible, as already noted in the context of constrained stochastic differential equations [4,5]. We show here how to enforce the numerical reversibility of the method to guaranteed that the sampling is unbiased. Our numerical results demontrate that this correction is indeed relevant in practice.

References

[1] S. Duane, A. Kennedy, B. Pendleton, D. Roweth, Physics Letters B, 195, 216-222 (1987)
[2] M. Girolami, B. Calderhead, Journal of the Royal Statistical Society: Series B (Statistical Methodology), 73, 123-214 (2011)
[3] B. Leimkuhler, S. Reich, Simulating Hamiltonian Dynamics, 2005
[4] E. Zappa, M. Holmes-Cerfon, J. Goodman, Comm. Pure Appl. Math., 71, 2609-2647 (2018)
[5] T. Lelièvre, M. Rousset, G. Stoltz, Numer. Math., 143, 379-421 (2019)

Passive particle in an active bath: can we tell it is out of equilibrium?

Jeanine Shea, Johannes Gutenberg University Mainz

We study a passive probe particle immersed in a fluid of active particles. In spite of the non-equilibrium nature of the system, the trajectory of the immersed particle does not exhibit obvious non-equilibrium signatures: its velocity distribution remains Gaussian, the second fluctuation dissipation theorem is not fundamentally violated, and the motion does not show apparent signs of time reversal symmetry breaking. To tell that the particle is out of equilibrium requires the examination of the behavior of the passive particle in tandem with the behavior of the active fluid: the kinetic temperature of the passive particle does not equilibrate to that of the surrounding active particles. Instead, it is higher, even though already the kinetic temperature of the active fluid is enhanced compared to the underlying thermal bath. As a strategy to diagnose non-equilibrium from analyzing probe particle trajectories only, we propose to examine their response to a small perturbation which reveals a non-equilibrium signature through a violation of the first fluctuation dissipation theorem.