Difficulty to capture non-additive enhancement of stacking energy by conventional ab initio methods

We evaluated the non-additive contributions in the inter-molecular interactions in B-DNA stacking by using diffusion Monte Carlo (DMC) methods. It is found that only DMC can capture the sign inversion in the contribution (i.e., the non-additivity enhances or reduces the interaction depending on the base pairs of DNA), which is never predicted by any other ab initio methods. Even by CCSD(T), the inversion is found to be difficult to be captured because of the practical handling of CBS (complete basis set correction) at the feasible level with MP2. The predicted non-additivity turns out to be several times larger (~8 kcal/mol) than those by other methods. The importance of the hydrogen bondings between the bases horizontally on the inversion is also clarified.