Improved Density-Functional Tight Binding Potentials For Metalloid Aluminum Clusters

LT Kim Joon, USN

In this thesis we study the feasibility of improving aluminum-carbon repulsive potentials for use in density-functional tight binding (DFTB) simulations of low-valence aluminum metalloid clusters. These systems are under consideration for use as novel fuels with rapid metal combustion kinetics, and contain an unusual mix of low-valence metal/metal bonds as well as organometallic components. We show that current DFTB parametrizations of the repulsive potential for Al/C interactions do not provide an adequate treatment of the bonding in these clusters. The repulsive term is the most difficult to fix in DFTB and contains a mix of ion-ion repulsion energy, exchange and correlation effects, and other physical quantities. We performed a re-parametrization of the Al-C repulsive potential via comparison to high-level density functional theory (DFT) results that are known to give accurate thermochemistry for these clusters. Additional manual adjustment of the potential was performed based on comparisons with experimental cluster structures. We found that the reparametrized system solves the most egregious issues, particularly those associated with an unphysical distortion of the η5 Al/cyclopentadienyl bond. DFTB molecular dynamics simulations of the oxidation of Al4Cp*4 show reasonable comparison with a DFT-based Car-Parrinello method, including correct prediction of hydride transfers from Cp* to the metal centers during the reaction.

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LT Kim Joon, USN


Jul 01, 2016

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