Actinide hydrates are incredibly complex systems to deal with computationally because of the presence of f-electrons, van der Waals interactions, and dynamic water. Density functional perturbation theory has been used successfully in the past to predict vibrational properties of static actinide solids with high symmetry. However, this method is less effective for more dynamic systems, such as hydrates with hydrogen-bonded water molecules that can rotate or translate through layers or pores. These mobile water molecules can affect various vibrational modes, especially in cases of strong hydrogen-bonding. For these complex hydrates, vibrational spectra can instead be predicted using Born-Oppenheimer ab initio molecular dynamic simulations. The system is evolved in time by performing a DFT calculation at each time step and then updating the atom positions based on the calculated forces. Once a trajectory has been calculated, the power spectrum showing all vibrational frequencies is obtained by the fast Fourier transform of the velocity autocorrelation function. Infrared and Raman spectra can then also be calculated by transforming the calculated electron density at each step from diffuse Bloch orbitals to maximally localized Wannier functions. The center position and spread of these localized functions can be used to track changes in the dipole moment and polarizability, allowing the infrared and Raman spectra to be obtained from the relevant autocorrelation functions. The utility of this methodology for complex systems has been demonstrated by predicting infrared and Raman spectra of multiple uranyl crystal hydrates of relevance to the uranium fuel cycle. Computational prediction of these spectra complements experimental work by informing band assignments and modeling how spectra may be expected to change various structural perturbations.

Predicting vibrational spectra of uranyl hydrates

255th National Meeting (2018)

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