Valentín-Rodríguez Mónica A, Bartolomei Massimiliano, Hernández Marta I, Campos-Martínez José, Hernández-Lamoneda Ramón
Centro de Investigaciones Químicas-IICBA, Universidad Autónoma del Estado de Morelos, Cuernavaca 62210, Morelos, Mexico.
Instituto de Física Fundamental, Consejo Superior de Investigaciones Científicas (IFF-CSIC), Serrano 123, 28006 Madrid, Spain.
J Chem Phys. 2021 Mar 14;154(10):104307. doi: 10.1063/5.0038926.
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 in 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)], 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 density functional theory (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 the 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 with spin S = 1 is retained within the tetramer unit vs another at higher pressures where the tetramer behaves as a single unit with a closed-shell character.
元素形式的氧在其凝聚相中表现出多种磁性特性；特别是，ε固相失去其磁性。这些现象反映了固体中存在的分子间力的性质以及随着压力和温度变化而产生的变化。在本研究中，我们使用通过无限制从头算方法获得的分子间势来模拟氧四聚体[(O)]的单重态，它是晶胞，与该相的非磁性特征一致。为此，我们对自旋算符的耦合 - 非耦合表示以及成对近似和海森堡哈密顿量进行分析。我们从在高水平计算的二聚体的无限制势以及不同的密度泛函理论（DFT）泛函开始，然后应用有限模型来预测ε相的性质。以这种方式获得的结果在低于60 GPa的整个压力范围内很好地再现了实验数据。此外，我们展示了计算四聚体单重态的重要性，这与之前的DFT周期性计算相反，在之前的计算中，无限制描述导致自旋态的混合且与实验的比较不佳。这一点在最近关于两个ε相共存的讨论中至关重要：一个是在四聚体单元内每个自旋S = 1的O的身份得以保留，另一个是在更高压力下四聚体表现为具有闭壳层特征的单个单元。
