Stacking Interactions of B-DNA and Their Nonadditivities: A QMC Study

Noncovalent interactions play significant roles in structures and functionality of biomolecules, but their accurate evaluation remains challenge in first-principles simulations. Fixed-node diffusion Monte Carlo (FNDMC) is one of the most promising approaches to tackle this problem in terms of computational costs and accuracy: It is known to be more accurate than MP2 (the second-order Møller-Plesset) and as accurate as "gold standard" quantum chemistry, CCSD(T)/CBS (coupled cluster including single, double, and noniterative triple excitations with complete basis set). Furthermore, FNDMC has a more desirable computational scaling (N^3-4) with respect to the number of electrons (N), compared to CCSD(T)/CBS (N⁷). With the help of modern supercomputers, we have applied FNDMC to ten unique B-DNA base-pair steps, for the first time, in order to evaluate their stacking energies. In addition, we evaluated their nonadditive (many-body effects) interaction energies (defined as the total stacking energy minus the sum of pair interaction energies) beyond the MP2 level (identical to SCF), which has not been investigated so far at the CCSD(T)/CBS level. Intriguingly, it is found that our “correlated” nonadditivities drastically change from the SCF/MP2 ones.