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. 2020 May 14;152(18):184304. doi: 10.1063/5.0005171.
The properties of molecular oxygen including its condensed phases continue to be of great relevance for the scientific community. The richness and complexity of its associated properties stem from the fact that it is a very stable diradical. Its open-shell nature leads to low-lying multiplets with total electronic spin S = 0, 1, 2 in the case of the dimer, (O), and the accurate calculation of the intermolecular potentials represents a challenge to ab initio electronic structure methods. In this work, we present intermolecular potentials calculated at a very high level, thus competing with the most accurate restricted potentials obtained to date. This is accomplished by drawing on an analogy between the coupled and uncoupled representations of angular momentum and restricted vs unrestricted methodologies. The S = 2 state can be well represented by unrestricted calculations in which the spins of the unpaired electrons are aligned in parallel; however, for the state where they are aligned in antiparallel fashion, it would seem that the total spin is not well defined, i.e., the well-known spin contamination problem. We show that its energy corresponds to that of the S = 1 state and perform unrestricted coupled cluster calculations for these two states. Then, we obtain the S = 0 state through the Heisenberg Hamiltonian and show that this is very reliable in the well region of the potentials. We make extensive comparisons with the best restricted potentials [Bartolomei et al., Phys. Chem. Chem. Phys. 10(35), 5374-5380 (2008)] and with reliable experimental determinations, and a very good agreement is globally found.
分子氧的性质，包括其凝聚相，对于科学界仍然具有重大意义。其相关性质的丰富性和复杂性源于它是一个非常稳定的双自由基这一事实。其开壳层性质导致在二聚体（O₂）的情况下存在低能多重态，总电子自旋S = 0、1、2，而分子间势能的精确计算对从头算电子结构方法构成了挑战。在这项工作中，我们展示了在非常高的水平上计算得到的分子间势能，从而与迄今为止获得的最精确的受限势能相媲美。这是通过利用角动量的耦合和非耦合表示与受限和非受限方法之间的类比来实现的。S = 2态可以通过非受限计算很好地表示，其中未成对电子的自旋平行排列；然而，对于它们反平行排列的状态，似乎总自旋定义不明确，即众所周知的自旋污染问题。我们表明其能量与S = 1态的能量相对应，并对这两个态进行了非受限耦合簇计算。然后，我们通过海森堡哈密顿量得到S = 0态，并表明在势能的阱区域这是非常可靠的。我们与最佳的受限势能[巴托洛梅伊等人，《物理化学化学物理》10(35)，5374 - 5380(2008)]以及可靠的实验测定进行了广泛比较，总体上发现了非常好的一致性。
