Grand canonical diffusion-influenced reactions: A stochastic theory with applications to multiscale reaction-diffusion simulations

Abstract

Smoluchowski-type models for diffusion-influenced reactions (A + B → C) can be formulated within two frameworks: the probabilistic-based approach for a pair A, B of reacting particles and the concentration-based approach for systems in contact with a bath that generates a concentration gradient of B particles that interact with A. Although these two approaches are mathematically similar, it is not straightforward to establish a precise mathematical relationship between them. Determining this relationship is essential to derive particle-based numerical methods that are quantitatively consistent with bulk concentration dynamics. In this work, we determine the relationship between the two approaches by introducing the grand canonical Smoluchowski master equation (GC-SME), which consists of a continuous-time Markov chain that models an arbitrary number of B particles, each one of them following Smoluchowski's probabilistic dynamics. We show that the GC-SME recovers the concentration-based approach by taking either the hydrodynamic or the large copy number limit. In addition, we show that the GC-SME provides a clear statistical mechanical interpretation of the concentration-based approach and yields an emergent chemical potential for nonequilibrium spatially inhomogeneous reaction processes. We further exploit the GC-SME robust framework to accurately derive multiscale/hybrid numerical methods that couple particle-based reaction-diffusion simulations with bulk concentration descriptions, as described in detail through two computational implementations.

Diffusion-influenced reaction rates in the presence of pair interactions

“…Collins and Kimball 68,69 refined Smoluchowski's model by introducing a finite rate at which molecules would react on contact. This model has been widely studied in the literature 58,59,70,71 , however, the singular nature of the reaction surface has drawbacks in computer simulations as the exact time of encounter is not resolved in a time-stepping algorithm. An alternative scheme was suggested by Teramoto and Shigesada 63 and further characterized by Doi [64][65][66] , which permits the reaction of two molecules with a microscopic rate , referred to as propensity 72 , as long as the reactants are within a reaction radius .…”