QMC Assessments of Weak-interaction Described by DFT within various XC approximations / Effects of Carbon Nanotube Oxida
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QMC Assessments of Weak-interaction Described by DFT within various XC approximations / Effects of Carbon Nanotube Oxidation on Molecular Interactions Yosuke Kanai, and Jeffrey C. Grossman Berkeley Nanosciences and Nanoengineering Institute (BNNI), University of California, Berkeley, Berkeley, CA, 94720 ABSTRACT 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 for those that are bound. 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 approximation 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 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 for the occasionallyobserved CNT conductance irreversibility in the experiment. INTRODUCTION We present in this proceeding two works (Parts 1 and 2) that address and exhibit the importance of weak interactions. The first part will discuss a fundamental aspect of molecular weak interactions described within various exchange correlation (XC) approximations in Density Functional Theory (DFT). The second part focuses on an application of DFT in describing various competing interactions between a single-walled carbon nanotube (CNT) and molecules, and how that changes under an oxidizing condition. Weak interactions in general play an important role in numerous chemical, physical and biological phenomena in nature. There are also enormous opportunities for using molecular weak interactions with carbon-sp2 nano-materials for various technological applications such as the hydrogen storage in CNTs/Fullerenes and CNT gas sensors. Although DFT calculations have been quite successful in describing covalent interactions, the non-covalent nature of weak interactions has been challenging to capture adequately within many current XC approximations in use. On the other hand, DFT has seen some successes in describing the trend on
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