On the Electronic Structure of the UO₂Molecule

Abstract

The structure and vibrational frequencies of the UO 2 molecule have been determined using multiconfigurational wave functions (CASSCF/CASPT2), together with a newly developed method to treat spin-orbit coupling. The molecule has been found to have a (5fφ)(7s), 3 Φ u , Ω ) 2 ground state with a U-O bond distance of 1.77 Å. The computed antisymmetric stretching σ u frequency is 923 cm -1 with a 16/18 isotope ratio of 1.0525 which compares with the experimental values of 915 cm -1 and 1.0526, respectively. Calculations of the first adiabatic ionization energy gave the value 6.17 eV, which is 0.7 eV larger than the currently accepted experimental result. Reasons for this difference are suggested.

A Fock space coupled cluster study on the electronic structure of the UO2, UO2+, U4+, and U5+ species

“…SOC is rather large, leading to significant admixture of 3 ⌬ u character in both the 2u ground state and the 3u first excited state ͑better described in a jj-coupling picture as pure 5f 5/2 1 7s 1/2 1 states͒. Actual calculations on gas phase UO 2 by Chang, 22 Gagliardi et al 4,18 and Fleig et al 23 reproduce this splitting well. The manifold of SOC-split grade states does not have two so closely spaced states at low energy.…”

“…Theoretical calculations 3 consistently gave a higher value. Gagliardi et al, 4 who had done accurate complete active space second order perturbation theory ͑CASPT2͒ calculations that gave an IP of 6.27 eV, proposed that the experimental data were in error. A new measurement by Han et al 5 using resonantly enhanced multiphoton ionization ͑REMPI͒, gave a value of 6.13 eV, in very good agreement with the theoretical values.…”

“…Since in these initial DFT calculations the effect of spin-orbit coupling ͑SOC͒ was neglected, theoreticians nevertheless questioned the validity of the simple picture presented. This spurred extensive theoretical work, notably by Gagliardi and co-workers, 4,[16][17][18] who applied the more sophisticated CASPT2 method and also studied the effect of SOC.…”

The Electronic Spectrum of Re2Cl82-: A Theoretical Study

“…19 It has been shown to be successful in the study of actinide compounds. 20,21 All calculations have been performed with the software MOLCAS-5. 22 A newly developed basis set of ANO type has been used for Re and Cl atoms.…”

On the reaction of a uranium atom with a nitrogen molecule: a theoretical attempt

“…[12][13][14] Relativistic effective core potentials (ECP) were used and no spin-orbit effects were included. The equilibrium structure of the OUO molecule was later determined 15 using the same approach together with a new method (CASSI-SO) to treat spin-orbit coupling. 16 The molecule was found to have a (5ff)(7s), 3 F u , O ¼ 2 ground state.…”