Brownian Motion and Microrheology in Complex Fluids under Periodic Boundary Conditions

We review the theoretical aspects of determining linear rheology using passive microrheology from simulations under periodic boundary conditions (PBCs). It is common to impose periodic boundary conditions when evaluating bulk properties by molecular simulation. The Brownian motion of the probe particles is affected by PBCs, and thus their effects must be considered when microrheological analysis is applied to evaluate the dynamic modulus. First, we outline the theory of microrheology based on the generalized Langevin equation (GLE) in an unbounded domain. Then, we briefly discuss the effect of PBCs on the diffusion coefficient. After that, we explain the effect of PBCs on the mean-square displacement of Brownian particles and their impact on microrheology. The passive microrheology technique under PBCs provides a new approach for rheology evaluation in molecular simulations of complex liquids in addition to the traditional Green-Kubo formula and non-equilibrium molecular dynamics (NEMD).