Ph.D. Thesis: Theoretical Actinide Chemistry – Methods and Models
by
Pernilla Wåhlin(Stockholm University, Department of Physics)
→
Europe/Stockholm
FA32
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Description
The chemistry of actinides in aqueous solution is important, and it is essential to build adequate
conceptual models and develop methods applicable for actinide systems. The complex electronic
structure makes benchmarking necessary. In the thesis a prototype reaction of the water exchange
reaction for uranyl(VI), for both ground and luminescent states, described with a six-water model,
was used to study the applicability of density functional methods on actinides and different
solvation models.
An excellent agreement between the wave function methods CCSD(T) and MP2
was obtained in the ground state, implying that near-minimal CASPT2 can be used with confidence
for the reaction in the luminescent state of uranyl(VI), while density functionals are not suited to
describe energetics for this type of reaction. There was an ambiguity concerning the position of
the waters in the second hydration sphere. This issue was resolved by investigating a larger model,
and prop- erly used the six-water model was found to adequately describe the water exchange
reaction. The effect of solvation was investigated by comparing the results from conductor-like
polarizable continuum models using two cavity models. Scattered numbers made it difficult to
determine which solvation model to use. The final conclusion was that the water exchange reaction
in the luminescent state of uranyl(VI) should be addressed with near-minimal CASPT2 and a
solvation model without explicit cavities for hydrogens. Finally it was shown that no new chemistry
appears in the luminescent state for this reaction.
The thesis includes a methodological investigation
of a multi-reference density functional method based on a range separation of the two-electron
interaction. The method depends on a universal parameter, which has been determined for lighter
elements. It is shown here that the same parameter could be used for actinides, a prerequisite for
further development of the method. The results are in that sense promising.
