  We introduce atomicrex, an open-source code for constructing interatomic
potentials as well as more general types of atomic-scale models. Such effective
models are required to simulate extended materials structures comprising many
thousands of atoms or more, because electronic structure methods become
computationally too expensive at this scale. atomicrex covers a wide range of
interatomic potential types and fulfills many needs in atomistic model
development. As inputs, it supports experimental property values as well as
\textit{ab initio} energies and forces, to which models can be fitted using
various optimization algorithms. The open architecture of atomicrex allows it
to be used in custom model development scenarios beyond classical interatomic
potentials while thanks to its Python interface it can be readily integrated
e.g., with electronic structure calculations or machine learning algorithms.
