Deviational methods are a class of stochastic particle simulation tools for solving kinetic equations. They have been developed [1][2][3] to alleviate the most important, perhaps, disadvantage of MC methods, namely their inability to efficiently simulate low-signal problems, such as small temperature differences. Deviational methods reduce statistical uncertainty by making use of deterministic information, a technique widely known as control-variate variance reduction [4]. In the case of kinetic equations, deviational methods make use of the observation that statistical noise becomes a limitation when transport signals are small, that is, the system state is close to equilibrium. By using that equilibrium state as a control, and using a stochastic method to simulate the deviation therefrom, deviational methods leverage exact solutions and focus the computational resources onto the unknown component of the phonon distribution function. As discussed further in Section 8, the ability to focus computational resources on the non-equilibrium component of the distribution function is very valuable for simulating multiscale problems.In the context of small-scale processes, the need for solving kinetic equations such as the Boltzmann transport equation (BTE), arises from the fact that the more tractable continuum description is only a limiting description valid in the limit of "small" mean free path, where transport is diffusive. Deviation from diffusive transport can be quantified by the Knudsen number,
AbstractWe review deviational methods for solving the Boltzmann equation governing phonon transport processes in the context of small-scale, solid-state heat transfer. We briefly discuss the numerical foundations of deviational algorithms as well as basic simulation methodology. Particular emphasis is given to recent developments in the field yielding appreciable efficiency improvements, such as linearized and adjoint formulations, and their applications to complex multiscale problems. Recently developed methods for simulating the ab initio collision operator for applications to phonon transport in novel two-dimensional materials, such as graphene, are also reviewed and discussed.