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.

CO saturation coverage on Pt(111) is crucially important in diesel oxidation catalysis. We systematically studied high coverage CO adsorption on the Pt(111) surface using density functional theory (DFT) calculations and classical Monte Carlo (MC) simulations. The zero-coverage limit CO adsorption energy at different binding sites is almost degenerate at the revised Perdew–Burke–Erzernhof functional (RPBE) level. As CO populates the surface, strong through-space repulsion and substrate-mediated metal sharing tends to dominate the stability of adsorbates and alter their binding preferences. The calculated differential binding energy curve and adsorption patterns compare well with experiments.

It is generally accepted that CO oxidation on transition metals follows a Langmuir-Hinshelwood mechanism. The oxidation reaction takes place in two sequential steps where the oxygen molecule first dissociates into atomic oxygen and then reacts with an adsorbed CO to form CO2. One critical question concerning the reaction kinetics under high pressure is the probability of oxygen dissociation on a highly CO covered surface. On bare transition metal surfaces, molecularly adsorbed oxygen readily dissociates with little or no apparent activation barrier. In industrial diesel engine catalysis, the metal surface is initially packed with CO. Subsequent reactions such as oxygen dissociation must take place on a CO covered surface. In this paper, we performed density functional theory (DFT) calculations for O2 dissociation on Pt(111) in the presence of different CO adsorption environments. While several stable O2 molecular precursor states (top-bridge-top, top-fcc-bridge, and top-hcp-bridge) exist on a clean Pt(111) surface, these precursors become endothermic beyond a critical CO coverage of ∼0.44 ML. Furthermore, the reaction path for CO oxidation via dissociated atomic oxygen becomes less favorable at higher CO coverage, primarily due to competitive adsorption and lateral repulsion. It was found that the oxygen dissociation barrier and the binding energies of atomic oxygen are well correlated via the Evans-Polanyi relationship.