The Molecular Oxygen Tetramer: Intermolecular Interactions and Implications for the $\epsilon$ Solid Phase

TL;DR

This paper investigates the structure and interactions of the oxygen tetramer in the high-pressure epsilon phase, revealing it as a van der Waals cluster with implications for understanding solid oxygen's properties.

Contribution

It provides a detailed ab initio characterization of the oxygen tetramer, clarifying its non-bonded nature and the role of many-body interactions in its stability.

Findings

Oxygen tetramer is a van der Waals cluster, not chemically bonded.

Exchange interactions stabilize the singlet ground state.

Many-body interactions soften the repulsive wall, affecting epsilon phase structure.

Abstract

Recent data have determined that the structure of the high pressure $\epsilon$ phase of solid oxygen consists of clusters composed of four O$_2$ molecules. This finding has opened the question about the nature of the intermolecular interactions within the molecular oxygen tetramer. We use multiconfigurational ab initio calculations to obtain an adequate characterization of the ground singlet state of (O$_2$)$_4$ which is compatible with the non magnetic character of the $\epsilon$ phase. In contrast to previous suggestions implying chemical bonding, we show that (O$_2$)$_4$ is a van der Waals like cluster where exchange interactions preferentially stabilize the singlet state. However, as the cluster shrinks, there is an extra stabilization due to many-body interactions that yields a significant softening of the repulsive wall. We show that this short range behavior is a key issue for the understanding of the structure of $\epsilon$-oxygen.