Book Description

This dissertation consists of two parts: quantum transitions (Part 1) and hydrodynamic limits of kinetic equations (Part 2). In both parts, we investigate the inner mathematical connections between equations for different physics at different scales, and use these connections to design efficient computational methods for multi-scale problems. Despite its numerous applications in chemistry and physics, the mathematics of quantum transition is not well understood. Using the Wigner transformation, we derive semi-classical models in phase space for two problems: the dynamics of electrons in crystals near band- crossing points; surface hopping of quantum molecules when the Born-Oppenheimer approximation breaks down. In both cases, particles may jump between states with comparable energies. Our models can capture the transition rates for such processes. We provide analytic analysis of and numerical methods for our models, demonstrated by explicit examples. The second part is to construct numerical methods for kinetic equation that are efficient in the hydrodynamic regime. Asymptotically, the kinetic equations reduce to fluid dynamics described by the Euler or Navier-Stokes equations in the fluid regime. Numerically the Boltzmann equation is still hard to handle in the hydrodynamic regime due to the stiff collision term. We review the theoretical work that links the two sets of equations, and present our asymptotic-preserving numerical solvers for the Boltzmann equation that naturally capture the asymptotic limits in the hydrodynamic regime. We also extend our methods to the case of multi-species systems.