For ECP and all-electron bases alike, using a smaller basis for geometry optimization (e.g. Def2SV) and then a larger for single point orbital and total energies (e.g. Def2TZVP), is a good way of obtaining good accuracy at a smaller computational cost.• None of these ECP bases include diffuse functions which are important for describing localized anions. In systems where oxygen or smaller moieties such as nitro groups bear formal negative charges, it is a good to use e.g. 6-31+G(d,p) for those corresponding atom types, and Def2SVP on remaining atom types.