We're happy to welcome:
Peter Bolhuis (University of Amsterdam)
Learning optimal reaction coordinates and models for activated processes in complex molecular systems
Molecular dynamics (MD) is a powerful tool for studying molecular processes but is limited by long timescales due to rare event dynamics. Enhanced sampling can address this by using a reaction coordinate (RC), which tracks the process progress. A well-chosen RC improves sampling and provides insights, with the optimal RC being the committor. However, finding this RC requires having sampled the rare transition sufficiently. This chicken-and-egg problem can be efficiently addressed by efficient Transition Path Sampling (TPS) methods and combining them with active learning [1,2]. While capable of effectively mapping the committor function the interpretability of this high dimensional function remains very low. Applying dimensionality reduction can reveal the RC in terms of low-dimensional human understandable molecular collective variables (CVs) or order parameters [1,3].
In the second part, I focus on a general framework for imposing known rate constants as constraints in molecular dynamics simulations, based on a combination of the maximum-entropy (MaxEnt) and maximum-caliber principles (MaxCal) to reweight rare event path ensembles [4]. The framework yields improved structure, kinetics and thermodynamics, and mechanistic insight that may not be readily evident directly from the experiments. As the method does not alter the Hamiltonian directly, we recently extended the MaxCal-based path-reweighting technique to optimize parameters in the molecular model itself, while constraining kinetic observables [5]. The methodology is illustrated on simple (colloidal) toy models. These developments open up the possibility to design molecular models that lead to desired kinetic behaviour and could open up a novel area of research, in which the optimization of kinetics and dynamical properties take center stage.
Sascha Jähnigen (Freie Universität Berlin)
Vibrational circular dichroism and multiscale chirality from first principles
Chiroptical spectroscopy offers an increasingly important, cost-effective alternative for the study of chiral compounds. In recent years, vibrational circular dichroism (VCD) - the chiral form of IR absorption spectroscopy - has emerged as a highly sensitive probe of molecular conformation and environment. VCD differs from electronic circular dichroism in that it relates directly to vibrational transitions in the (supramolecular) chiral framework, such as functional groups linked by covalent or non-covalent interactions. The interpretation of VCD spectra requires accurate calculations that yield the magnetic response of the electronic structure associated with the nuclear motion. After a brief introduction to the theoretical background, I present results obtained specifically for the solid state, identifying unique non-local patterns that can be attributed to chiral crystal packing and space group symmetry. I show that solid-state VCD is highly sensitive to the enantiomorphism of molecular crystals and can be used to access the different levels of chirality at the molecular and supramolecular level.
Sabine Klapp (Technische Universität Berlin)
Self-organization in nonreciprocal active matter
Recently much attention has been devoted to self-organization of soft-matter systems that are intrinsically out of equilibrium. A prime example are systems of “active” particles that perform persistent motion, but also particle systems with non-reciprocal couplings generated by a nonequilibrium environment. In this talk I will discuss the impact of nonreciprocity on self-organization in active and passive systems on various scales.
The first example concerns mixtures of repulsive active particles with non-reciprocal polar alignment. We investigate the collective behavior of these systems using a combination of mean-field-like continuum theory, particle-based simulations of the underlying Langevin equations, and a corresponding fluctuation analysis. We show that nonreciprocity has profound influence already below the threshold related to spontaneous time dependency of polarization dynamics. In particular, nonreciprocal alignment alone can induce asymmetrical density dynamics, where single-species clusters chase more dilute accumulations of the other species. As a second example, we consider systems of passive patchy particles that, in equilibrium, form rigid clusters. By considering a binary version we show that nonreciprocity can have an annealing effect, allowing the system to escape kinetic traps.