Fast and accurate quasi-atom method for simultaneous atomistic and continuum simulation of solids

• First demonstration of a hybrid multiscale approach for simulation collisions of micron-size particles’, while maintaining atomic-level accuracy in the contact region. • Elaboration of an efficient AI-based optimization scheme to determine parameters of interaction potentials between quasi-atoms, comprising composite medium, which mimics continuum medium with the same material constants. • Elaboration of Python-based package for LAMMPS for a seamless optimization of quasi-atom parameters and multiscale MD simulations. • Verification of Hertz and JKR theories for impacts of micron-size particles with pairwise and many-body inter-atomic potentials. We report a novel hybrid method of simultaneous atomistic simulation of solids in critical regions (contacts surfaces, cracks areas, etc.), along with continuum modeling of other parts. The continuum is treated in terms of quasi-atoms of different size, comprising composite medium. The parameters of interaction potential between the quasi-atoms are optimized to match elastic properties of the composite medium to those of the atomic one. The optimization method coincides conceptually with the online Machine Learning (ML) methods, making it computationally very efficient. Such an approach allows a straightforward application of standard software packages for molecular dynamics (MD), supplemented by the ML-based optimizer. The new method is applied to model systems with a simple, pairwise Lennard-Jones potential, as well with multi-body Tersoff potential, describing covalent bonds. Using LAMMPS software we simulate collision of particles of different size. Comparing simulation results, obtained by the novel method, with full-atomic simulations, we demonstrate its accuracy, validity and overwhelming superiority in computational speed. Furthermore, we compare our method with other hybrid methods, specifically, with the closest one – AtC (Atomic to Continuum) method. We demonstrate a significant superiority of our approach in computational speed and implementation convenience. Finally, we discuss a possible extension of the method for modeling other phenomena.