在紫外线辐照下，不同结构的水（包括水分子团簇、液态水、冰）中水分子的光解是自然界重要的光化学过程，其中还有很多悬而未决的问题。本申请拟利用我们开发的多体格林函数理论激发态动力学模拟程序研究不同结构的水在激发态的性质和动力学过程，以增进对水光解机制的理解。这方面的研究包含两部分，（1）不同尺寸和构型的水分子团簇中水分子在不同波长紫外线辐照下的光解过程；（2）液态水和冰的光吸收谱与激子性质，液态水和冰中的缺陷与表面对激发态的影响，以揭示液态水和冰的光吸收谱中Urbach tail的来源及其对光解的贡献。在计算激发态作用力时，我们目前的程序采用了有限差分法。为了进一步提高计算精度和效率，在该项目中，我们也将发展一种在多体格林函数理论框架内解析计算激发态作用力的新方法，该方法将采用在coupled perturbed Hartree-Fock理论基础上发展起来的Z-vector方法。

Photodissociation of water molecules in water clusters, liquid water and ice under ultraviolet irradiation is an important photochemical process in nature. There are still many open questions remaining to be solved on the photolysis of water. In this project, we will study the excited-state properties and dynamics in water clusters, liquid water and ice using the many-body Green’s function theory (MBGFT) excited-state dynamics simulation code developed by ourselves, in order to improve our understandings on the mechanism of water photolysis. The research on this respect includes two parts, (1) photolysis of water molecules in water clusters of different sizes and shapes by ultraviolet irradiations at different wavelengths; (2) optical absorption spectra and exciton properties of liquid water and ice, the effects of defects and surfaces on the excited states of liquid water and ice, in order to reveal the origin of Urbach tails in the absorption spectra of liquid water and ice and its contribution to the photolysis of water. Finite-difference method is used to compute excited-state forces in our present MBGFT code. In order to increase both the calculation precision and efficiency, we will also develop a new analytic approach to calculate excited-state forces within MBGFT. This new approach is based on the so-called Z-vector method which was developed from the coupled perturbed Hartree-Fock theory.