THERMACOND, a Computational Code for Lattice Thermal Conductivity from Harmonic and Anharmonic Force Constants

THERMACOND is an open-source software developed for ab initio calculations of phonon thermal transport in crystalline bulk materials, utilizing both the relaxation time approximation (RTA) and the exact solution of the phonon Boltzmann transport equation (PBTE) via iterative or direct methods. Its main inputs are harmonic and anharmonic interatomic force constants (IFCs), which can be extracted from first-principles force-displacement calculations using FOCEX, a code that is also part of the Anharmonic Lattice Dynamics (ALADYN) suite of codes. THERMACOND optimizes the computational time needed to achieve a satisfactory level of convergence by exploiting crystal symmetry to solve the PBTE over the irreducible wedge of the Brillouin zone (IBZ) rather than the full Brillouin zone (FBZ). Given the numerical instabilities associated with the iterative approach, we present a direct noniterative approach to enhance calculation efficiency, which has none of the related numerical instabilities. It also incorporates the tetrahedron method for precise Brillouin zone integrations of δ functions arising from energy conservation.
THERMACOND, available under Gnu public license GPLv3, is written in Fortran90 and parallelized with MPI to manage the computational costs, making it suitable for large k-point meshes in large crystals. Here, we provide an overview of its key theoretical components and program structure, showcasing THERMACOND's capabilities through two case studies: Germanium (Ge) and Germanium Selenide (GeSe). The strong agreement of the results with both experimental data and previous theoretical studies highlights the software's ability to handle materials with varying symmetries and structural complexities.