碱基(腺嘌呤A，鸟嘌呤G，胞嘧啶C，胸腺嘧啶T)是DNA基本和重要的生色基团。研究DNA碱基的激发态性质及其光物理过程对理解其超快光循环过程具有重大意义。本文以腺嘌呤(A)为研究实例，利用完全活化空间自洽场(CASSCF)方法并结合多组态二级围绕理论(CASPT2), 得到了腺嘌呤基态及6个电子激发单态S3N(1nπ*)、S1N(1nπ*)、SLa(1ππ*)、SLb(1ππ*)、S7N(1nπ*)及S9N(1nπ*)和三态T3N(3nπ*)、T1N(3nπ*)、TLa(3ππ*)、TLb(3ππ*)、T7N(3nπ*)及T9N(3nπ*)的极小几何构型。同时计算了腺嘌呤分子的吸收光谱，发现其最强的吸收峰在215nm处，将其指认为SLb(1ππ*)激发。这一计算结果很好的解释了实验观察的UV 吸收光谱。同时利用CASSCF态平均方法优化得到了2个1nπ*态与基态的势能面交叉点，S3N(1nπ*)/S0和S7N(1nπ*)/S0。这些交叉点在腺嘌呤(A)的超快光循环过程中发挥重要作用。

DNA base (adenine (A), guanine (G), cytosine (C), thymine (T)) is a basic and important chromophore. The investigations on the nature of the excited states and photophysical processes of DNA base have great significance for understanding of its ultrafast-photo-cycle. In present thesis, we selected adenine (A) as an example using Complete Active Space Self-Consistent Field method (CASSCF) followed by Second order multi-configurational perturbation theory (CASPT2). The minima of adenine on different electronic states (S3N(1nπ*), S1N(1nπ*), SLa(1ππ*), SLb(1ππ*), S7N(1nπ*) and S9N(1nπ*); T3N(3nπ*), T1N(3nπ*), TLa(3ππ*), TLb(3ππ*), T7N(3nπ*), T9N(3nπ*)) were optimized. The absorption spectra of adenine were also compted by CASSCF/CASPT2 calculations. It was found that the strongest absorption peak localized at 215 nm that was assigned to be 1ππ* transition (SLb(1ππ*)). These findings well elucidated the experimental observation of the UV absorption spectra. The conical intersections between two 1nπ* and ground state were found by state-averaged CASSCF optimizations. These critical poins could play an important role in ultrafast photo-cycle of DNA base.