Sella is an open source Python software package that is available on both github.com and from the Python Package Index (PyPI). Additionally, the exact version of Sella used to run these calculations is available from Zenodo. Listing 1 contains a minimal Python script that uses Sella to perform saddle point optimization on a molecule in an internal coordinate basis. An initial geometry contained in input_structure.xyz is read using ASE's IO routines. An ASE calculator for NWChem is attached to the resulting ASE Atoms object using all default settings, which will perform a calculation at the HF/3-21G level of theory. This level of theory can be changed by adding arguments to the NWChem calculator object constructor, and the code used to perform the calculation can be altered by replacing the NWChem class with a different calculator for an alternative code. A Sella optimizer object is then created for the ASE Atoms object, with optimization in internal coordinates requested explicitly. Finally, the optimization to a saddle point is run with a convergence criteria of a maximum atomic force of 0.01 eV Å -1 .