An unrestricted approach for the accurate calculation of the intermolecular potential of (O2)4: Implications for the solid epsilon phase

Abstract

Oxygen in its elemental form shows a variety of magnetic properties in its condensed phases, in particular the epsilon solid phase loses its magnetism. These phenomena reflect the nature of the intermolecular forces present in the solid and the changes that arise with variations of pressure and temperature. In this study we use intermolecular potentials obtained with unrestricted ab initio methods to model the singlet state of the oxygen tetramer((𝑂 2 ) 4 ), which is the unit cell, consistent with the non-magnetic character of this phase. To do this, we perform an analysis of the coupled-uncoupled representations of the spin operator together with a pairwise approximation and the Heisenberg Hamiltonian. We start from unrestricted potentials for the dimer calculated at a high level as well as different DFT functionals and then apply a finite model to predict the properties of the epsilon phase. The results obtained in this way reproduce well the experimental data in the entire pressure range below 60 GPa. Additionally, we show the importance of calculating the singlet state of tetramer as opposed to previous DFT periodic calculations where the unrestricted description leads to a mixture of spin states and a poor comparison with the experiment. This point is crucial in the recent discussion about the coexistence of two epsilon phases: one where the identity of each 𝑂 2 with spin 𝑆 = 1 is retained within the tetramer unit versus another at higher pressures where the tetramer behaves as a single unit with a closed shell character.

Molecular Oxygen Trimer: Multiplet Structures and Stability

“…[19,20] This phase has several unique properties such as a magnetic and volumetric collapse with increasing pressure. [21,22] In fact, a simple model based on a tetramer interaction potential plus pairwise interactions [23,24] can accurately reproduce the pressure dependence of the structural parameters as measured experimentally.…”