Relativistic coupled-cluster calculations on XeF6: Delicate interplay between electron-correlation and basis-set effects

A systematic relativistic coupled-cluster study is reported on the harmonic vibrational frequencies of the Oh, C-3v, and C-2v conformers of XeF6, with scalar-relativistic effects efficiently treated using the spin-free exact two-component theory in its one-electron variant (SFX2C-1e). Atomic natural orbital type basis sets recontracted for the SFX2C-1e scheme have been shown to provide rapid basis-set convergence for the vibrational frequencies. SFX2C-1e as well as complementary pseudopotential based computations consistently predicts that both Oh and C3v structures are local minima on the potential energy surface, while the C2v structure is a transition state. Qualitative disagreement between the present results for the Oh structure and those from CCSD(T)-F12b calculations [ Peterson et al., J. Phys. Chem. A 116, 9777 (2012)], which yielded a triply degenerate imaginary frequency for the Oh structure, is attributed here to the high sensitivity of the computed harmonic frequencies of the t(1u) bending modes to the basis-set effects of triples contributions. (C) 2015 AIP Publishing LLC.