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AB INITIO STUDIES OF THE DIURANIUM MOLECULE

Please use this identifier to cite or link to this item: http://hdl.handle.net/1811/18181

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Title: AB INITIO STUDIES OF THE DIURANIUM MOLECULE
Creators: Pepper, M.; Bursten, Bruce E.; Pitzer, Russell M.
Issue Date: 1990
Abstract: The question of the existence and nature of uranium-uranium multiple bonds has been investegatedvia an initio self-consistent field (SCF), multiconfiguration SCF (MCSCF)and configuration interaction (CI) calculations on the diuranium molecule. Relativistic effects have been included using effective core petentials. In the LS coupling regime, the potential surface for $U_{2}$ shows a double minimum similar to that reported for the dichromium molecule. At short bond distances (2.2 \AA), both 5f and 6d atomic orbitals are involved in the formation of molecular orbitals in $U_{2}$. The lowest-energy state is $^{5}\Sigma_{g}{^{+}}$, with configuration $d\pi_{u} ^{4}f\delta_{\Sigma} {^{2}}d\delta_{g}\delta{^{2}}\delta\sigma^{2}$. At longer bond distances (3.0.3.6 \AA), the 5f atomic orbitals remain atom-like, with each atomic f shell containing three electrons. The remaining six valence electrons occupy molecular orbitals formed from 6d and $^{7}s$ atomic orbitals, resulting in a group of high-spin states which are closely spaced in energy. The bonding in the these states is principally through the $^{7}s\sigma_{g}$ and $6d\pi_{u}$ molecular orbitals. Considering the 5f core as a pair of $^{4}S$ states coupled $^{7}\Sigma_{g}{^{+}}$, all possible occupancies of the $^{6}d$ and $^{7}s$ molecular orbitals have been considered. $9\Sigma_{g}{^{-}}, 9\Sigma_{u} +, ^{11}\Sigma_{g} \Delta_{g} ^{9}\Pi_{u}$ and $^{13}\Delta_{u}$ states are of roughly equivalent energies, and all display shallow potential minima near 3.0{\AA}. At the SCF and CAS-MCSCF levels of theory, the 2.2 \AA states lie approximately 250 kcal/mol higher in energy than the high-spin 3.0 \AA states. Inclusion of correlation effects through single-reference Cl calculations reduces the energy difference to approximately 120 kcal/mol, with the high-spin states remaining lower in energy. The relative energies of the two sets of states should be further affected by inclusion of spin-orbit and additional correlation effects, providing an even more accurate description of the $U_{2}$ molecule
URI: http://hdl.handle.net/1811/18181
Other Identifiers: 1990-RD-8
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