Basis Set Incompleteness Errors in Fixed-Node Diffusion Monte Carlo Calculations on Noncovalent Interactions

Basis set incompleteness error (BSIE) is a common source of error in quantum chemistry calculations, but it has not been comprehensively studied in fixed-node Diffusion Monte Carlo (FN-DMC) calculations. FN-DMC, being a projection method, is often considered minimally affected by basis set biases. Here, we show that this assumption is not always valid. While the relative error introduced by a small basis set in the total FN-DMC energy is minor, it can become significant in binding energy (Eb) evaluations of weakly interacting systems. We systematically investigated BSIEs in FN-DMC-based Eb evaluations using the A24 data set, a well- known benchmark set of 24 noncovalently bound dimers. We found that BSIEs in FN-DMC evaluations of Eb are indeed significant when small localized basis sets, such as cc-pVDZ and cc-pVTZ, are employed. Our study shows that the aug-cc-pVTZ basis set family strikes a good balance between computational cost and BSIEs in the Eb calculations. We also found that augmenting the basis sets with diffuse orbitals, using counterpoise correction, or both, effectively mitigates BSIEs, allowing smaller basis sets such as aug-cc-pVDZ to be used.