镧系锕系化合物的化学性质的探索对于理解重元素的周期律、核能的开发和利用具有重大意义，其研究工作长期以来是放射化学领域的研究热点。锕锕分离和镧锕分离是目前放射化学学科的基础科学问题。溶剂萃取法是实现湿法锕锕分离和镧锕分离的重要技术，因此如何认识镧系和锕系离子在复杂溶液化学环境中的化学行为显得尤为重要。理论化学作为化学研究的重要工具，可以用于模拟镧系锕系离子在复杂溶液化学环境中的化学行为。本文在现有电子相关理论和相对论理论及其计算方法的基础上，对镧系锕系化合物开展了系统的理论研究，同时开发了可用于第一性原理的从头算分子动力学模拟(AIMD)的赝势，并将它们应用到溶液环境下的理论计算和模拟。本论文主要开展了以下三个方面的研究工作:一、系统地研究了锕系元素的高价氧化态行为，揭示了锕系元素周期律。基于密度矩阵重整化群（DMRG）和多组态自洽场（MCSCF）方法我们研究了一系列AnO4 (An = Ac-Cm)化合物的电子结构和氧化态渐变规律。锕系元素从Ac到Np，其最高氧化态等于其外层电子数；从Pu开始，锕系元素的最高氧化态远低于其外层电子数，而超锔元素的最高价氧化态将会表现出低价氧化态III价。镧系锕系元素的氧化态变化规律对理解镧系锕系元素的化学性质具有重要意义，同时也为第二部分赝势的可移植性测试提供了重要的依据。二、系统地开发了镧系锕系元素的相对论性Geodecker, Teter和Hutter (GTH)赝势和MOLOPT高斯基组。镧系锕系离子溶液化学的模拟需要相应的分子动力学研究技术，AIMD是镧系锕系溶液化学研究的重要手段，然而镧系锕系元素目前仍然缺乏有效的赝势和基组，从而阻碍了其研究工作的开展。Geodecker, Teter和Hutter提出的GTH赝势是一个可分离的高斯型的赝势，具有较高的准确性和计算效率。我们对镧系锕系元素拟合了精确的GTH赝势和基组，为f区元素化合物的AIMD模拟和理论计算奠定了基础。 三、系统地研究了三价镧系离子在水溶液中的物理化学行为。在GTH赝势和基组研究的基础上，我们对三价镧系离子水溶液体系开展了第一性原理分子动力学模拟。探讨了三价镧系离子水合物的结构、扩散性质以及光谱性质，为镧系锕系配位化学研究提供了新的思路。
The f-block elements of lanthanides and actinides are essential in fundamental chemistry and nuclear technology. The exploration of lanthanide and actinide chemistry is the central theme in nuclear and radio-chemistry field due to the application of lanthanide and actinide compound in nuclear energy. The actinide/actinide separation and lanthanide/actinide separation are the two fundamental problems in radiochemistry. Solvent extraction is a promising method to realize the actinide separation and lanthanide/actinide separation. It is therefore important to understand the chemical behavior of lanthanide and actinide ions in complex solution. Theoretical chemistry based on quantum mechanics plays an important role in modern chemistry research. One can model the chemical behaviors of lanthanide and actinide ions in complex solutions from the first-principles calculations. Based on the current ab initio electron correlation and relativistic quantum-chemical methods, we have performed high-level ab initio calculations for lanthanide and actinide compounds and developed the pseudopotentials and relevant basis sets for lanthanides and actinides. They can be used in both modeling of electronic structures and ab initio molecular dynamics (AIMD) simulations in solution. Our research focus on the following three aspects: 1) We have systematically studied the high oxidation states of actinide elements in several selected actinide compounds. Our study enriches the understanding of periodicity of actinide elements. We performed ab initio calculations for AnO4 (An = Ac-Cm) molecules by using density matrix renormalization group (DMRG) and multi-configuration self-consistent field (MCSCF) method. It shows that the oxidation state equals to the available number of valence electrons from Ac to Np, and the oxidation state is not significantly lower than the available valence-electron number starting from Pu. For late lanthanides, the low oxidation state III is common. We try to explain this phenomenon from the orbital energy difference and interactions between metal and ligand. The rule of oxidation state variation of actinide elements is of great importance to understanding the chemical behavior of actinide elements. Moreover, it provides theoretical basis for us to test the transferability of pseudopotentials of lanthanides and actinides in the subsequent study.2) We systematically develop the relativistic Geodecker, Teter and Hutter (GTH) pseudopotentials and accompanying MOLOPT Gaussian basis sets. The molecular dynamics method is needed to simulate the lanthanide and actinide solution chemistry. AIMD method is an important and powerful technique to study the lanthanide and actinide speciation in solution. However, the AIMD simulations for lanthanide and actinide system are limited due to the lack of efficient and accurate pseudopotentials and basis sets. The GTH pseudopotential formalism proposed by Geodecker, Teter and Hutter provides an accurate separable pseudopotential. These works provide new avenue to perform molecular dynamics simulations for lanthanide and actinide systems.3) We also systematically study the physical and chemical behavior of trivalent lanthanide ions in aqueous solution by using our pseudopotentials and basis sets. We will summarize in this section the theoretical results of the structures, the diffusion coefficient and the spectrum of lanthanide ion hydrate. This work provides new insights for studying lanthanide and actinide coordination chemistry.
