量子多体计算方法研究钴氧复合物的低能谱

Water splitting process under cobalt-based water oxidation catalyst is a promising way to convert sunlight into fuels. Direct experimental assignment of these electronic levels has been impossible, because they lie at low energies and can be embedded within the vibrational modes of the complexes. Therefore, it is important to have an investigation from theoretical point of view. Their electronic structure, often characterized by high-spin-polarized metal sites, is a challenge because of their high degree of dynamical, static correlation and the exchange interactions between the Cobalt atoms. In this project, we develop and use the Extended Broken Symmetry approach to construct the Hamiltonian and derive approximate ground-state energies and, for the first time, forces for the correctly symmetric ground state of an arbitrary number of spin centers. Based on Resonating Valence Bond theory, we constructed the wavefunction and then optimize it under Quantum Monte Carlo method in order to study the electronic structure of this complex. By studying systematically the low-lying energy spectrum, we expect our results will give a deep understanding the way that the cobalt-based water oxidation catalysts determine the water splitting process and contribute to the theoretical foundations and improving the efficiency of the artificial photosynthesis.

钴氧复合物催化水分解是人工光合作用中一种极具潜力的方式。低能激发态是此复合物反应活性的关键。实验上，由于振动谱的干扰，此复合物的低能谱难以直接测量，所以采用理论计算方法研究其电子结构就有着重要意义。理论上，由于多个钴原子之间局域自旋的交换作用形成的静态和动态电子关联，使得很难进行计算模拟。在本项目中，我们发展扩展对称破缺方法来构造哈密顿量描述自旋交换作用，进而研究此体系的的低能激发态对应的能级分布以及振动谱。同时基于共振价键理论构造多体波函数，采用量子蒙特卡罗方法进行优化以研究此体系；通过分析上述方法获得的能谱结构，我们期待理解钴氧复合物催化水分解机制，为模拟光合作用和提高效率提供理论依据。

过渡金属复合物丰富的能谱和振动谱结构是诸多系统比如光反应中心、生物酶催化中心等有较强化学活性的根本原因。无论是理论方法，还是实验方法对其丰富的结构进行研究都是一个难点。在本项目中，我们发展扩展对称破缺方法来构造哈密顿量描述自旋交换作用，进而研究此体系的的低能激发态对应的能级分布以及振动谱。获得了包括钴氧复合物在内的过渡金属体系的丰富能谱结构。在研究过渡金属复合物系统沿着势能面进行几何结构优化问题时，发现此体系的基态能级对应的本征态不是自旋量子数最小的量子态而是多重简并态。此发现有助于对光合作用和生物酶催化机理的理解提供帮助。为了解决上述问题，我们在原有扩展对称破缺方法基础上进行了算法拓展，为了能够在任意自旋的多重简并态下对过渡金属复合物体系进行几何结构优化，进而得到结构振动谱为研究过渡金属复合物系统Jahn-Teller效应提供了可能。算法实现的程序源码继续在Github上开源供审阅和参考。