We evaluated the non-additive contributions of the inter-molecular interactions in B-DNA stacking by using diffusion Monte Carlo methods with fixed node approximations (FNDMC). For some base-pair steps, we found that their non-additive contributions evaluated by FNDMC significantly differ from those by any other {it ab initio} methods, while there are no remarkable findings on their stacking energies themselves. The apparently unexpected results of non-additivity raise issues in both FNDMC and correlated wavefunction methods. For the latter, it can be partly attributed to the imperfect complete basis set (CBS) correction scheme due to the limitation of the computational costs. On the other hand, the striking contrast between the stacking and non-additivity behaviors was found in FNDMC. This might imply that the error cancellations of the fixed node biases in FNDMC work well for the stacking energies, while not for the non-additivity contributions involving charge transfers caused by hydrogen bonds bridging Watson-Crick base pairs.
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