Coupling ESPResSo and waLBerla for scalable soft matter simulations with hydrodynamic interactions

Many problems in soft matter research involve particles suspended in a solvent, where hydrodynamic interactions play a crucial role. A well-established simulation approach for such systems combines molecular dynamics for the particles with the lattice-Boltzmann method for the solvent. We present the coupling of the coarse-grained molecular dynamics code ESPResSo to the waLBerla library, a high-performance framework for lattice-Boltzmann and other stencil-based methods. This coupling code, developed in the context of the EuroHPC Centre of Excellence MultiXscale, enables large-scale, multi-GPU simulations of coupled particle–fluid systems within ESPResSo. Beyond hydrodynamics, the integration also provides a diffusion–advection–reaction solver coupled to electrostatics and lattice-Boltzmann for electrokinetic simulations. A key advantage of using waLBerla is its code generation infrastructure, which facilitates both the adaptation of algorithms and their optimisation for different hardware architectures. We demonstrate the scalability of coupled simulations with multi-GPU benchmarks on MareNostrum 5.