DEVELOPMENT OF MOLECULAR THEORY FOR THE CALCULATION OF POTENTIAL ENEGY SURFACES

单参考微扰理论和团簇展开理论如对称适应团簇理论和耦合团簇理论在描述动态关联方面是非常有效的，但在处理非动态关联方面却很失败。CI很容易应用于多参考形式，并且可以处理非动态相关，但是随着系统中电子数量的增加，CI展开变得不那么紧凑和低效。因此，我们发展了基于多参考的瑞利-薛定谔摄动理论。MRMP是可靠的，并保留了单一参考MP方法的吸引人的特点。由于采用独立的电子对模型，该理论具有概念上的简单性。它的大小几乎一致。这是非常高效和成本效益。多参比法可以正确地将分子分解成其片段。它适用于开壳层和激发态。状态特异性MRMP已成功地应用于化学反应势能面的计算。该理论还应用于苯的化合价能和里德堡激发能的研究。计算结果与实验结果吻合较好。计算得到的价态π - π ^*激发能（圆括号内的实验值）分别为^1B_<2u>,4.77 (4.90), ^1B_<1u>,6.28 (6.20), ^1E_<1u>,6.98（6.94）和^1E_<2g>,7.88 (7.80) eV。里德堡激发能的预测精度达到0.18 eV或更高。对环戊二烯、呋喃、吡咯、萘和臭氧的化合价能和里德堡激发能也得到了类似的精度。

Single reference perturbation theory and cluster expansion theory such as symmetry adapted cluster and coupled cluster theories are very effective in describing dynamical correlation but fail badly in dealing with nondynamical correlation. CI is easily applied in multireference form and can handle nondynamical correlation but the CI expansion becomes less compact and less efficient as the number of electrons in the system grows. Therefore we have developed multireference based Rayleigh-Schrodinger perturbation theory. The MRMP is reliable and retains the attractive features of the single reference MP method. The theory has conceptual simplicity due to the independent electron pair model. It is almost size consistent. It is very efficient and cost effective. Multireference technique can dissociate a molecule correctly into its fragments. It is applicable to open shells and excited states.The state-specific MRMP has been sucessfully applied to the calculations of the potential energy surfaces of chemical reactions. Also the theory was applied to the study of valence and Rydberg excitation energies of benzene. The results compare well with the experiment. The calculated valence pi-pi^* excitation energies (experimental values in parentheses) are ^1B_<2u>,4.77 (4.90), ^1B_<1u>,6.28 (6.20), ^1E_<1u>,6.98 (6.94) and ^1E_<2g>,7.88 (7.80) eV,respectively. The Rydberg excitation energies are also predicted with an accuracy of 0.18 eV or better. Results of similar accuracy are obtained for the valence and Rydberg excitation energies for cyclopentadiene, furan, pyrrole, naphthalene and ozone.