Benchmarking Basis Sets for Density Functional Theory Thermochemistry Calculations: Why unpolarised basis sets and the polarised 6-311G family should be avoided

Basis sets are a crucial but often largely overlooked choice when setting up quantum chemistry calculations. The choice of basis set can be critical in determining the accuracy and calculation time of your quantum chemistry calculations. Clear recommendations based on thorough benchmarking are essential, but not readily available currently. This study investigates the relative quality of basis sets for general properties by benchmarking basis set performance for a diverse set of 136 reactions (from the diet-150-GMTKN55 dataset). In our analysis, we find the distributions of errors are often significantly non-Gaussian, meaning that the joint consideration of median errors, mean absolute errors and outlier statistics is helpful to provide a holistic understanding of basis set performance. Our direct comparison of performance between most modern basis sets provides quantitative evidence for basis set recommendations that broadly align with the established understanding of basis set experts and is evident in the design of modern basis sets. For example, while zeta is a good measure of quality, it is not the only determining factor for an accurate calculation with unpolarised double and triple-zeta basis sets (like 6-31G and 6-311G) having very poor performance. Appropriate use of polarisation functions (e.g. 6-31G*) is essential to obtain the accuracy offered by double or triple zeta basis sets. In our study, the best performance in our study for double and triple zeta basis set are 6-31++G** and pcseg-2 respectively. The polarised 6-311G basis set family has poor parameterisation which means its performance is more like a double-zeta than triple-zeta basis set. All versions of the 6-311G basis set family should be avoided entirely for valence chemistry calculations moving forward.