单线态裂分是指有机材料在光激发后，从一个单重态激子转变为两个三重态激子的光物理 过程。该过程有望提高光电转换效率，因而受到了广泛的关注。研究发现单线态裂分中存在一个 关键的中间态，而如何构建中间态的波函数则是重要的挑战。本文着重介绍了双激发态波函数构 建的两类理论模型，而后讨论了振动、轨道和自旋相互作用对中间态形成和解离的影响。最后总 结了进一步的理论研究将面临的挑战。 

Singlet fission is a photophysical process in organic materials where a photoexcited singlet exciton splits into two triplet excitons. This process has gained a lot of interest in the past decade for enhancing the efficiency of photovoltaic conversion. Precious work identified a crucial intermediate state in singlet fission, and the construction of the wave function for this correlated triplet-pair state poses a significant challenge. This article serves as a comprehensive introduction to theoretical models designed for constructing the wave function of the correlated triplet-pair. Subsequently, we delve into an exploration of the intricate effects of vibrational, orbital, and spin interactions on the formation and dissociation of the intermediate state. Finally, we conclude with a succinct summary of the challenges anticipated to shape the landscape of future theoretical research in this field.