The diffusion Monte Carlo (DMC) method is generally considered less sensitive to basis set incompleteness than conventional electronic structure approaches. Its performance for systems containing second-row elements in high oxidation states—where tight d-functions play an essential role—remains insufficiently explored. In this work, we investigate the influence of basis sets and high-exponent d-functions on atomization energies of molecules containing first- and second-row elements using both DMC and CCSD(T) with correlation-consistent effective core potentials. Tight d-functions are found to significantly affect the nodal structure of the trial wave functions, particularly for molecules containing second-row elements in high oxidation states, thereby impacting the DMC energies. For molecules containing second-row elements, at least the a(T+d)Z or aQZ basis set is required to obtain accurate results, whereas molecules containing second-row elements in high oxidation states demand even larger sets such as a(Q+d)Z or a5Z. Moreover, basis set effects on DMC energies are approximately half those observed at the HF level, with a strong correlation between the two, suggesting that HF calculations can provide useful guidance for basis set selection in DMC. These findings highlight the critical role of tight d-functions in ensuring reliable DMC predictions for chemically challenging systems.
Huang et al. (Mon,) studied this question.