Modelling novel resonant X-ray spectroscopy for transition metal biological complexes and nano-particles

这是一项理论建议，旨在发展新的数值方法来描述材料和分子中的量子激发及其非平衡动力学。该项目带来了Cedric Weber博士在分子过渡金属系统的量子建模方法方面的互补专业知识，以及Rafael T. M. de Rosales博士在医学成像背景下使用的功能纳米颗粒方面的专业知识。在这里，我们将模拟在硬核x射线光谱中诱导的核孔状态的动力学，用于探测分子和纳米级系统的化学和磁性。学生将开发一种新的建模方法来描述典型的X射线光谱方法，如X射线吸收光谱（XAS），并将形式主义扩展到共振非弹性X射线散射（RIXS）模型。一个规范的系统，光系统II (PSII)，将在整个项目中考虑。PSII已经被研究了几十年，但是支撑水分解机制的机制仍然缺乏。建模困难源于过渡金属原子（四个锰原子）引起的非平凡量子物理，这需要最先进的方法。第一套理论技术将基于密度泛函理论（DFT）和时变DFT (TD-DFT)[1]，它解释了x射线和岩心-孔场的动态筛选。特别是，TD-DFT的一个典型问题是多重态的存在，学生将探索DFT的扩展，以获得DFT[2]中多确定性效应的准确描述。然后，学生将该方法扩展到纳米级系统，并特别关注超磁氧化铁纳米颗粒（SPIONs）。SPIONs具有广泛的应用，特别是在磁共振成像（MRI）中用作造影剂。在这个项目中，学生将专注于理解纳米粒子中的磁激发，以及未来如何通过RIXS技术解决这些问题。纳米粒子的磁矩弛豫是一个复杂的失平衡过程，动力学与构象性质之间存在着微妙的相互作用。特别是，由于它们的空间缩减而产生的量子约束有望提供新的和复杂的非平衡性质，因为外部磁场被淬灭。还有许多主要的未解问题，如外层断裂键引起的磁挫折的作用，以及纳米粒子核心与涂层之间的配体键的作用。该项目的目的是通过优化纳米颗粒的磁弛豫率来指导更好的造影剂的设计。事实上，目前的一个限制是，大的纳米颗粒具有最佳的磁弛豫率，但由于表面效应往往会聚集，这阻碍了它们的磁矩弛豫和它们作为造影剂的应用。我们在这里提供了一个基于量子从头算模拟的一致理论，包括动力学效应。该项目还与钻石光源设施的新x射线光束线的建设有关，这将是探测这些新材料，以及化学和生物化学中广泛的分子和其他重要材料的激发的关键。

This is a theoretical proposal aiming at developing novel numerical approaches to describe quantum excitations and their out-of-equilbrium dynamics in materials and molecules. This project brings the complementary expertise from Dr Cedric Weber on quantum modeling approaches for molecular transition metal systems, and Dr Rafael T. M. de Rosales on functional nano-particles used in the context of medical imaging.Here, we will model the dynamics of a core-hole state induced in hard core X-ray spectroscopy, used to probe chemical and magnetic properties of molecules and nano-scopic systems.The student will develop a new modeling approach to describe typical X-ray spectroscopy methods, such as X-ray absorption spectroscopy (XAS) and extend the formalism to model resonant inelastic X- ray scattering (RIXS). A canonical system, photo-system II (PSII), will be considered throughout the project. PSII has been investigated for decades, but the mechanism underpinning the water-splitting mechanism is still lacking. The modeling difficulties stem from the non-trivial quantum physics induced by the transition metal atoms (the four Mn atoms), which require state-of-the-art approaches. The first set of theoretical techniques will be based on density functional theory (DFT) and time-dependent DFT (TD-DFT) [1], which accounts for the dynamic screening of the X-ray and the core-hole fields. In particular, a typical problem for TD-DFT is the presence of multiplets, and the student will explore extensions of DFT to obtain an accurate descriptions of multi determinantal effects in DFT [2]. The student will then extend the methodology to nano-scopic systems, and in particular focus on super pagramagnetic iron oxide nano-particles (SPIONs). SPIONs have a range of applications, in particular they are used as contrast agents in magnetic resonant imaging (MRI). In this project, the student will focus at understanding magnetic excitations in nano-particles, and how they could be resolved in the future via the RIXS technique. The relaxation of the magnetic moment of nano-particles is a complex out-of-equilibrium process, and there is a subtle interplay between the dynamics and the conformation properties. In particular, the quantum confinement stemming from their spatial reduction is expected to provide new and complex out-of-equilibrium properties, as the external magnetic field is quenched. There are many major unanswered questions, such as the role of the magnetic frustration induced by the broken bonds in the outer-layers, and the role of the ligand bonding between the core of the nano-particle and the coating. The purpose of the project is to guide the design of better contrast agents, via optimizing the magnetic relaxivities of the nano-particle. Indeed, a current limitation is that large nano-particles have optimal magnetic relaxivities, but tend to agglomerate due to surface effects, which hinders the relaxation of their magnetic moments and their application as contrast agents. We provide here a consistent theory based on quantum ab-initio simulations, including dynamical effects.This project is also connected to the construction of a new X-ray beam line at the Diamond Light Source facility, that will be key to probe excitations in these new materials, as well as in a wide range of molecules and other materials of importance in chemistry and bio-chemistry.