大共轭分子的电子相关效应研究-从静态描述到动态模拟

项目名称： 大共轭分子的电子相关效应研究-从静态描述到动态模拟

项目编号： No.20873058

项目类型： 面上项目

立项/批准年度： 2009

项目学科： 建筑科学

项目作者： 刘春根

作者单位： 南京大学

项目金额： 31万元

中文摘要： 半经验量子化学方法是从头计算法的必要补充，具有模型简单，物理图像清晰，结果定性正确、计算量相对较小等诸多优点。本项目发展了基于不等性杂化原子轨道的MM/VB模型，实现了平面与非平面共轭分子的统一描述，能够获得基态与激发态的能量及其梯度。对于共轭分子骨架因张力与空间位阻效应所造成的非平面扭曲也能正确描述。结合DMRG方法，可以实现纳米尺度一维与准二维共轭大分子的计算，获得正确的结构、能量与波函数等重要信息。基于MM/VB模型，发展了一种非绝热近似分子动力学方法，可以描述处于激发态的共轭分子的态-态转变。该方法基于始态与终态电子波函数的同时含时演化与分子骨架的经典力学轨迹描述。在锥形交叉点附近，VB模型的电子波函数具有多参考性质，且能够表达始态与终态的非绝热耦合。考虑到VB模型哈密顿的紧邻近似特点，电子波函数含时演化计算采用新近发展的Split Operator近似算法，使电子波函数时间演化的计算量仅随共轭键数目而近似线性增加。上述计算方案中避免了能量本征值的计算，使计算效率进一步提高，从而可以对更高的激发态和更大的体系进行非绝热分子动力学模拟。

中文关键词： 半经验量子化学方法； 密度矩阵重整化群；非绝热分子动力学;激发态； 共轭分子

英文摘要： Semi-empirical quantum chemical method usually has the joint merits of simple, physically clear, qualitatively correct, computationally less resources demanding, etc, which always make it a necessary complimentary to ab initio methods. In this project, a new MM/VB model has been developed using hybrid atomic orbital as atomic bases, which permits a universal description for planar and non-planar pi-conjugated molecules, both the ground state and the excited states. The non-planar distortions due to the steric effect are also well accounted. Combined with the DMRG method, the MM/VB model is applicable to nano-scale 1-D and quasi 2-D conjugated molecules like polyphenylene and graphene nanoribbons. Besides, a new non-adiabatic molecular dynamics method has been developed to deal with the state-to-state transitions in the excited states of conjugated systems, using the MM/VB model. This method is based on the simultaneous time-evolution of the intial and target electronic states, which are represented with the VB model, as well as classical trajectory description of the vibrational motions of the molecular backbone in terms of molecular mechanics. Our method is benefit from the multi-reference character of the VB model, which could give qualitatively correct description of the non-adiabatic coupling of the two states, which are essential for the computations near conical intersections. Thanks to the nearest-neighbor approximation in the VB model, the newly developed SPLIT OPERATOR technique has been adopted to greatly cut down the time-cost for the time-evolution of the electronic wavefunctions. With the SPLIT OPERATOR technique, the time consumption increases approximately linearly with the number of conjugated chemical bonds. Avoid of eigenvalue computation further improved the efficiency of the method, which makes it possible for the non-adiabatic molecular dynamics simulation for larger systems.

英文关键词： Semiempirical quantum chemistry method;DMRG;non-adiabatic molecular dynamics; Excited state; Conjugated molecules