Elucidating electronic structure using single point and spectral simulation calculations

这个项目的目的是探索金属-硅混合团簇的电子结构，特别是探索金属-金属和金属-硅键的变化对实验观察值的影响。在过去的十年里，人们从实验和计算两个角度探索了硅的面内团簇的光谱和电子结构，最终目的是了解金属的电子性质如何影响团簇，反之亦然。除了对这些团簇中化学键性质的内在兴趣外，它们还可以被视为块状硅中过渡金属杂质的最小模型，这在半导体行业具有相当大的商业意义。因此，该项目属于EPSRC物理科学研究领域。直接感兴趣的是物种，如ReSix+，它们正在Fielicke(柏林)和Janssens(鲁汶)的团队中进行研究，我们曾与他们合作解决涉及较轻元素的问题。团簇是在气相中产生的，通过多光子解吸技术间接测量了它们的振动光谱。因此，要解决的第一个问题是，原子的几何排列是什么。我们的作用是使用密度泛函理论，并最终使用从头算理论，来回答这个问题，首先是Re团簇，然后是更多样化的过渡金属基团簇。理论对此提供了两种方法--计算能量是稳定性的直接测量，原则上，计算能量最低的异构体应该是实验中存在的异构体。然而，密度泛函理论(DFT)往往很难回答这个简单的问题--不同的泛函(密度和能量之间的不同关系)往往会给出非常不同的相对能量估计，特别是当自旋态或共价键的性质发生变化时。这个问题可以通过应用从头算理论来解决，例如耦合团簇(CCSD(T))，这个项目的一个重要元素将是扩展到DFT之外。在这种规模和复杂性的系统上进行CCSD(T)计算仍然具有挑战性，我们对这个问题的初步探索表明，基集收敛和Hartree-Fock参考的质量需要仔细考虑。另一种方法是计算光谱性质，并尝试确定与实验数据的良好匹配：如果感兴趣的性质(在这种情况下，振动光谱)的计算对方法变化的敏感度低于总能量，这可能是一种更稳健的方法。通过仔细的分析，尽可能地利用对称性，我们已经能够解码像Mn2Si12这样的团簇的振动光谱，使得数据比简单的指纹更有用。最终，我们的目标是从现有的实验数据中提取关于金属-金属和金属-硅键的信息。这项工作本质上是协作的，我们打算与Janssens和Fielicke团队密切合作，以最大限度地发挥我们方法的影响。

The purpose of this project is to explore the electronic structure of mixed metal-silicon clusters, and in particular to explore how changes in metal-metal and metal-silicon bonding influence experimental observables. Over the past decade the spectroscopy and electronic structure of endohedral clusters of silicon has been explored from both experimental and computational perspectives, the ultimate goal being to understand how the electronic properties of the metal impact on the cluster and vice versa. Beyond the intrinsic interest in the nature of the chemical bonds in these clusters, they can viewed as minimal models for transition metals impurities in bulk silicon, an issue of considerable commercial significance in the semi-conductor industry. Therefore, this project falls within the EPSRC Physical Sciences research area. The clusters of immediate interest are species such as ReSix+ which are being studied in the groups of Fielicke (Berlin) and Janssens (Leuven), with whom we have collaborated on problems involving lighter elements. The clusters are generated in the gas phase and their vibrational spectra measured indirectly through multi-photon desorption techniques. The first question to address is, therefore, what the geometric arrangement of atoms is. Our role is to use density functional theory and, ultimately, ab initio theories, to answer this question for the Re clusters in the first instance, and later more diverse transition-metal based clusters. Theory offers two approaches to this - the computed energy is a straightforward measure of stability, and, in principle, the isomer with the lowest computed energy should be the one that is present in the experiment. However, density functional theory (DFT) often struggles to answer this apparently simple question - different functionals (different relationships between density and energy) can often give very different estimates of relative energies, particularly when spin states or the nature of the covalent bonding changes. This problem can be addressed by applying ab initio theory such as coupled-cluster (CCSD(T)), and an important element of this project will be to extend beyond DFT. CCSD(T) calculations on systems of this size and complexity remain challenging, and our initial exploration of this issue suggests that basis set convergence and the quality of the Hartree-Fock reference require careful consideration. An alternative approach is to compute spectroscopic properties and try to identify a good match to experimental data: this is, potentially, a more robust approach if the computation of the property of interest (vibrational spectra, in this case) is less sensitive to changes in methodology than the total energy. Through careful analysis, using symmetry as far as possible, we have been able to decode the vibrational spectra of clusters such as Mn2Si12, making the data much more useful than simply a fingerprint. Ultimately, we aim to extract information about metal-metal and metal-silicon bonding from the available experimental data. The work is intrinsically collaborative, and we intend to work closely with the Janssens and Fielicke groups to maximise the impact of our approach.