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，如对称性自适应团簇理论和耦合团簇理论，在描述动力学关联时非常有效，但在处理非动力学关联时效果很差。CI很容易以多参考形式应用，并可以处理非动力学关联，但CI展开随着系统中电子数量的增加而变得不那么紧凑和效率更低。因此，我们发展了基于多参考的瑞利-薛定谔微扰理论。该方法是可靠的，并保留了单参考MP方法的优点。由于独立的电子对模型，该理论具有概念上的简单性。它的尺寸几乎是一致的。这是非常有效率和成本效益的。多重参考技术可以正确地将一个分子解离成它的片段。该方法适用于开壳和激发态，并已成功地应用于化学反应势能面的计算。并将该理论应用于苯的价态和里德堡激发能的研究。计算结果与实验结果吻合较好。计算价pi-pi^*激发能(括号内实验值)分别为^1B_&lt；2U&gt；，4.77(4.90)，^1B_&lt；1U&gt；，6.28(6.20)，^1E_&lt；1U&gt；，6.98(6.94)和^1E_&lt；2g&gt；，7.88(7.80)eV。里德堡激发能的预测精度也达到了0.18 eV或更高。对环戊二烯、呋喃、吡咯、萘和臭氧的价激发能和里德堡激发能也得到了类似精度的结果。

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.