### About

Atomistic simulations of materials rely on the description of the interatomic interactions. A good description of the interatomic interaction is often provided by quantum mechanical methods such as density functional theory (DFT) or more accurate quantum chemical methods. However, the applicability to extended time scales and length scales is usually limited due to the computational effort. This can be partly overcome by coarse-graining the description of the electronic structure that governs the interatomic interactions.

In a first step of coarse graining, DFT is simplified to the Tight-Binding (TB) approximation. This comprises a formal expansion of the DFT functional with respect to charge fluctuations and the representation of the DFT wavefunction in an optimized atomic-like basis. The resulting energy is then represented in the TB bond model, which provides a transparent and intuitive framework for modelling the interatomic interaction. In a second step of coarse graining a general, local approximation to the solution of the TB bond model is constructed in terms of the analytic bond-order potentials (BOPs). With the resulting functional form the analytic BOPs comprise covalent bond formation, charge transfer and magnetism.

The BOPfox program provides an implementation of real-space analytic BOPs and reciprocal-space TB. The analytic BOP implementation provides linear scaling of the computation time for energies and forces with the number of atoms. The different parallelisations of BOPfox allow to run the calculations with optimum use of the hardware resources for a given problem size. The program can be compiled as standalone program or as library with an API for linking with an external software (ASE, openKIM, LAMMPS).