Smaller and more efficient yet still accurate basis sets are particularly rare for heavier elements like lanthanides and actinides. To the best of our knowledge, q-vSZP is the first minimal basis set to cover these elements consistently. It is crucial to ensure that both qualitative and, where possible, quantitative accuracy are preserved since SQM methods using q-vSZP otherwise have to compensate for potential basis set limitations. A relevant test case is U2C@C80 (cf. to Fig. ), a diuranium cluster stabilized inside a C 80 fullerene cage. By assessing its interaction energy, which involves a mixture of covalent and non-covalent contributions, 37,96 also BSSE and effects dominated by polarization and more diffuse functions are probed. Calculating the interaction energy with ωB97X-D4 39 using a standard triple-ζ basis set such as def2-TZVP 89 (def-TZVP for uranium) yields an interaction energy of -268.7 kcal mol -1 .