Hartree−Fock Orbital Instabilities and Symmetry-Breaking in ScO2: Is aCsEquilibrium Structure Viable?

Abstract

We have applied high-level coupled cluster methods, in conjunction with a variety of reference molecular orbitals and basis sets, to consider the possibility that the equilibrium geometry of the ground state of ScO 2 breaks C 2 v symmetry. The force constants for the antisymmetric stretching vibration (b 2 symmetry) have been computed across a domain of Sc-O bond distances and O-Sc-O bond angles at the spin-restricted open-shell Hartree-Fock (ROHF) and spin-unrestricted Hartree-Fock (UHF) levels of theory in order to investigate the importance of artifactual orbital instability envelopes on the properties computed with correlated wave functions. In most cases, Hartree-Fock instability regions are located far from the pertinent optimized geometries, suggesting that the corresponding harmonic vibrational frequencies should be free from artifactual orbital effects. Nevertheless, ROHF- and UHF-based coupled cluster models disagree qualitatively on the symmetry of ScO 2 , and Brueckner orbital based methods give variable results with respect to basis set and level of electron correlation. Although full coupled cluster single-, double-, and triple-excitation results indicate symmetry breaking with smaller basis sets, extrapolation of the results to larger basis sets is inconclusive. The current results indicate with certainty only a flat symmetry-breaking potential. Furthermore, although all methods considered here predict that C s optimized structures lie lower in energy than their C 2 v counterparts, the highest levels of theory predict very low effective barriers to interconversion of equivalent C s minimalow enough that the zero-point vibrational energy (even when computed with anharmonicity corrections) lies above the barrier leading to an overall dynamical C 2 v symmetry.