This paper first presents the fixed-node diffusion and reptation quantum Monte Carlo (QMC) assessment of density functional theory (DFT) description of weak molecular interactions within several exchange-correlation (XC) approximations. The case of Benzene-molecule complex was studied with a number of small molecules. We found that the binding energy for the set of complexes depend significantly on XC approximation employed although its trend appears rather consistent. Analysis of the reduced density gradient revealed that the binding energy trend among different XC approximation is highly correlated with the exchange enhancement factor behaviors at large reduced density gradients. Our findings point to the importance of accurate exchange energy functional in describing the weak interactions, in addition to the more obvious correlation part in DFT description.

We then present DFT investigation on molecular interaction of NO3 toward a single-wall carbon nanotube (CNT). We aim to explain the atomistic processes that are responsible for the recently observed CNT conductance changes in the redox process. We found that molecular physisorption and chemisorption to be energetically stable states, and the chemisorbed configuration becomes less endothermic with an increased oxidation state of the CNT. Our results also show that the dissociative incorporation of the molecule can become highly favored energetically in the process. This chemical state may play an important role of the occasionally-observed CNT conductance irreversibility in the experiment.