光系统I（PSI）反应中心的激子耦合

运用点偶极、单极跃迁电荷和跃迁密度等经典库仑作用的方法，考察了叶绿素a分子间面心距和错位结构等因素对激子耦合的影响．结果表明，当分子间距大于分子尺寸时，上述三种方法得到的结果基本一致；但当分子间距小于分子尺寸时，点偶极方法将明显高估激子耦合，跃迁密度的方法更适合计算分子间的激子耦合．此外，还使用上述方法计算了光系统Ⅰ（PSI）反应中心叶绿素a分子间激子耦合．结果表明，用跃迁密度计算PSI晶体结构（1jb0．pdb）e700的激子耦合为75.3cm^-1，而QM．MM优化的结构P700（ecAl．ecBl）的激子耦合为23．8cm^-1，这与考虑Dexter交换项的全对角化方法的结果（20cm^-1）一致，进而说明PSI反应中心并不是传统的P700强激子耦合对，而是ecAl-ecB2和ecBl-ecA2对强耦合二聚体构成的二聚体对．

Exciton coupling in the photosystem I reaction center

The point dipole approximation, transition charge method and transition density cube method are used to investigate the influences of the packing structures such as face-to-face distance and shift distance on the exciton coupling between theChlorophyll-a model dimers. The results show that the coupling strength obtained from three methods are very similar when the distance between the dimer molecules is larger than the molecule size; while the point dipole approximation will overestimate the exciton coupling and transition density cube method will give more reasonable coupling values when the distance is smaller than the molecule size. Based on these three method, we also calculated the couplings between Chlorophyll a pigments in both the crystal structure （ljb0.pdb） and the QM-MM optimized structure of the photosystem I reaction center. The exciton coupling of the special pair （P700） from the transition density cube method is 23.8 cm^-1 for optimized structure, which is very similar with the one 20 cm^-1 from full INDO Hamiltonian decomposition including F6rster and Dexter term contributions. This explained that the strongest coupling dimer in the PSI reaction center is not the P700, but the dimer pair of the ecAl-ecB2 （-153 cm^-1） and ecB 1-ecA2 （-152 cm^-1）.