Electronic structure investigation of the evanescent AtO(+) ion

The electronic structure of the XO and XO(+) (X = I, At) species, as well that of a AtO(+)-H2O complex have been investigated using relativistic wave-function theory and density functional theory (DFT)-based approximations (DFAs). The n-electron valence state perturbation method with the perturbative inclusion of spin-orbit coupling including spin-orbit polarization effects (SO-NEVPT2) was shown to yield transition energies within 0.1 eV of the reference four-component intermediate Fock-space coupled cluster (DC-IHFSCCSD) method at a significantly lower computational cost and can therefore be used as a benchmark to more approximate approaches in the case of larger molecular systems. These wavefunction calculations indicate that the ground state for the AtO(+) and AtO(+)-H2O systems is the Ω = 0(+) component of the (3)Σ(-) LS state, which is quite well separated (by ≃0.5 eV) from the Ω = 1 components of the same state and from the Ω = 2 state related to the (1)Δ LS state (by ≃1 eV). Time-dependent DFT calculations, on the other hand, place the Ω = 1 below the Ω = 0(+) component with the spurious stabilization of the former increasing as one increases the amount of Hartree-Fock exchange in the DFAs, while those employing the Tamm-Dancoff approximation and DFAs not including Hartree-Fock exchange yield transition energies in good agreement with SO-NEVPT2 or DC-IHFSCCSD for the lower-lying states. These results indicate the ingredients necessary for devising a DFA-based computational protocol applicable to the study of the properties of large AtO(+) clusters so that it may (at least) qualitatively reproduce reliable reference (SO-NEVPT2) calculations.