Time-dependent density functional theory(TD-DFT) is extensively applied to calculate the excited state of pi-conjugated molecules. But the conventional functional will underestimate the energies of excited states with charge transfer character. A new density functional, Coulomb-attenuated hybrid excharge-correlation functional(CAM-B3LYP) has recently been developed to overcome these limitations. In this paper, the four lowest excited states of chlorophyll-a were calculated by accurate SAC-CI( symmetry adapted cluster coupled configuration interaction), ZINDO (Zerner' s intermediate neglect of differential overlap) and various time-dependant DFT' methods. The influences of substituents and Nstate on the excited states of chlorophyll-a were discussed. The orientation of three-dimensional transition dipole moment of the Qy band was assigned by different methods. Among different DFT methods, CAM-B3LYP is the best method to describe the four lowest excited states of chlorophyil-a. The CAM-B3LYP results are in excellent agreement with accurate SAC-CI resuits.%采用密度泛函理论DFT(B3LYP/6-3G)对氧取代后叶绿素的几何构型进行优化,并用不同的量子化学方法如ZINDO-CIS,TD-DFT和SAC-CI计算激发态能量,考察叶绿索的侧链和单线态根数目对激发态能量的影响,并计算Q,态跃迁偶极矩与y轴的夹角.结果表明,CAM-B3LYP泛函形式是计算叶绿素a前4个激发态特征的最佳方法,与高精度方法SAC-CI的结果是一致的.
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