  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.
