  Advances in nanotechnology have facilitated the exploration of submicron
thermal transport. At this scale, Fourier's law is no longer applicable, and
the governing equation for thermal transport is the phonon Boltzmann transport
equation (BTE). However, the availability of open-source solvers for the phonon
BTE is limited, impeding progress in this field. This study introduces an
open-source package, GiftBTE, for numerically solving the non-gray phonon BTE.
GiftBTE employs deterministic solutions and provides both steady-state and
transient solvers. For the steady-state solver, GiftBTE employs the implicit
discrete ordinates method (DOM) with second-order spatial accuracy and the
synthetic iterative scheme. For the transient solver, GiftBTE employs the
explicit DOM with second-order spatial accuracy. This package demonstrates
excellent computational efficiency, enabling realistic three-dimensional
simulations of devices and materials. By interfacing with first-principles
calculations, this solver enables parameter-free computation of submicron
thermal transport. The application of GiftBTE includes, but is not limited to,
computing the thermal conductivity of nanostructures, predicting temperature
rises in transistors, and simulating laser heating processes.
