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 (<i>E</i>_b) evaluations of weakly interacting systems. We systematically investigated BSIEs in FN-DMC-based <i>E</i>_b 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 <i>E</i>_b 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 <i>E</i>_b 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.