Benchmarking electronic structure calculations on the bare UO2(2+) ion: how different are single and multireference...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy of Actinides

Background:

• Discrepancies exist between CASPT2 and linear response coupled cluster (LRCC) methods for uranyl (UO22+) ion spectra.
• The reliability of these methods for actinide-containing molecules is questioned, particularly concerning spin-orbit coupling effects.

Purpose of the Study:

• To compare the accuracy of CASPT2 and LRCC methods for actinide spectroscopy.
• To evaluate the performance of these methods against the four-component intermediate Hamiltonian Fock-space coupled cluster (IHFSCC) method.

Main Methods:

• Utilized the four-component intermediate Hamiltonian Fock-space coupled cluster (IHFSCC) method as a benchmark.
• Compared CASPT2 and LRCC calculations for the uranyl (UO22+) ion.
• Analyzed the impact of electron correlation and spin-orbit coupling on spectral descriptions.

Main Results:

• Electron correlation, rather than spin-orbit coupling, is identified as the main source of discrepancies between methods.
• IHFSCC and LRCC show strong agreement in describing excited states, with minor blue-shifts in LRCC energies.
• CASPT2 agrees with IHFSCC on individual frequencies but less so on the overall spectrum due to differences in recovered correlation.

Conclusions:

• For uranyl ions, IHFSCC and LRCC provide reliable descriptions of electronic spectra.
• CASPT2 may be less suitable for comprehensive actinide spectral analysis compared to LRCC and IHFSCC.
• Accurate treatment of electron correlation is crucial for high-fidelity actinide spectroscopy.