Making Complete-Basis-Set Extrapolation Visible: A Spreadsheet-Based Activity for Upper-Division Computational Chemistry

Complete-basis-set (CBS) extrapolation is widely used to estimate electronic energies beyond finite-basis calculations. However, students may encounter it primarily as a formula or automated correction, which obscures how a CBS value is constructed from a sequence of finite-basis energies. We describe a scaffolded Excel activity that makes this process visible by linking energy tables, component extrapolations, and convergence plots. Students first examine Hartree-Fock and correlation-energy convergence in the hydrogen chloride heteronuclear diatomic molecule, compare CBS estimates obtained from alternative extrapolation choices, and then apply the same workflow to the hydrogen fluoride molecule HF. The activity culminates in the calculation and justification of CBS-derived atomization energies and comparison to experiment. This activity is implemented in an upper-division computational chemistry course, and was evaluated through a pre-activity quiz and rubric-based analysis of completed student work. In general, students executed the component extrapolations successfully, yet evaluating sensitivity and justifying a final reported value proved more challenging. The activity offers a transparent and adaptable approach for teaching students to evaluate, rather than merely obtain, computational chemistry results.