Effects of basis sets and tight d-functions in quantum Monte Carlo and CCSD(T) calculations with pseudopotentials

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.