UNO CI studies of nonlinear optical properties and excited staes of unstable molecules and clusters

在这个研究项目中，我们的UNO-CI计算方法已经被纳入到AlChemy II程序包中，用于计算不稳定分子体系的势能曲线、激发能、跃迁几率等。用UNO CI方法描述了线性HN体系和三重亚甲基二聚体的势能曲线。还对这些系统进行了UNO CASSCF计算。已经确定，UNO-CI波函数是计算UNO CASSCF的一个很好的尝试。其次，为了研究分子和分子团簇在强激光作用下的频变非线性光学响应，提出了一种新的方法--数值刘维尔方法(NLA)。对NLA方法的数值稳定性和计算可靠性进行了详细的检验。用NLA方法计算了多烯的超极化率。计算结果与在非共振区和共振区的实验结果一致。为了阐明决定其晶体结构的因素，对DNBB的簇合物进行了从头算分子轨道计算。研究发现，氢键相互作用是控制晶体结构的最重要因素。结果表明，氢键的有效利用对分子基材料的晶体工程方法至关重要。

In this research project, our UNO-CI computation method has been incorporated into the Alchemy II program package in order to calculate potential curves, excitation energies, transition probabilities, etc.for unstable molecular systems. The UNO CI method has been applied to depict the potential curves of the linear Hn systems and the dimer of triplet methylene. The UNO CASSCF calculations have also been carried out for these systems. It has been established that the UNO-CI wavefunction is a very good trial for the UNO CASSCF calculation. Thus, the computation scheme without external electromagnetic fields has been established.Next, a new method, numerical Liouville approach (NLA), has been proposed in order to investigate the frequency-dependent nonlinear optical responce of molecules and molecular clusters under the strong laser fields. The numerical stability and computational reliability of the NLA method have been examined in details. The NLA method has also been applied to calculate the hyperpolarizabilities of polyenes. The results are in consistent with the experiments in both the off-resonant and resonant regions.Ab initio MO calculations have been performed for clusters of DNBB in order to elucidate factors which determine its crystal structure. It has been found that the hydrogen bonding interaction is the most important factor to control the crystal structure. It is concluded that the effective use of hydrogen bondings is essential for crystal engineering approach towards molecular-based materials.