质子化和低能电子俘获对N-糖苷键解离性质影响的理论研究

采用B3LYP密度泛函方法,在6-311++G(2d,2p)基组水平上,对3种嘌呤核苷体系及其N7-质子化和低能电子俘获形成的衍生物结构性质、键解离能、水解机理进行了计算调查.计算结果显示,质子化嘌呤核苷与中性嘌呤核苷的水解机理并不相同,且质子化可以明显减少N-糖苷键离子解离通道所需要的能量,降低其水解活化能,稳定水解产物,极大地促进N-糖苷键的水解.与质子化作用类似,嘌呤核苷俘获低能电子,也能显著地降低N-糖苷键的键解离能,显著地影响嘌呤核苷的稳定性.     

Qualitatively graph-theoretical study on stability and formation of fullerenes and nanotubes

Kekule structures of different carbon species have been determined. On the basis of Kekule structure and C-C bond counts as well as the surface curvature, stability of diverse carbon species, driving force for curling of graphite fragments and formation of fullerenes and nanotubes, have been discussed. Curling of graphite flat fragments, end-capping of nanotubes, and closure of curved structures are driven by a tremendous increase in Kekule structures as terminal carbon atoms couple their dangling bonds into C-C o bonds. The increasing tendency becomes particularly striking for large cages and nanotubes. Resonance among numerous Kekule structures will stabilize the curved structure and dominate formation of closed carbon species. For similar carbon cages with comparable Kekule structure counts in magnitude, the surface curvature of carbon cages, as a measure for the strain energy, also plays an important role in determining their most stable forms.     

用变分Monte Carlo方法处理分子

对变分量子Ｍｏｎｔｅ Ｃａｒｔｏ方法提出了一种种算法：将传统的Ｈａｒｔｒｅｅ－Ｆｏｅｋ方法与量子Ｍｏｎｔｅ Ｃａｒｌｏ方法有机结合在一起；导出了“局部能”的解析式；使用了一种新的相关函数和随机数发生器。我们用这个新算法计算了Ｈ２、ＬｉＨ、Ｌｉ２、Ｈ２Ｏ、Ｆ２分子的基态和ＣＨ２分子的＾３Ｂ１、＾１Ａ１态的能量。计算结果表明，这个新算法在精度和统计误差两个方面比一般ＶＭＣ过程都要好得多。     

稳定的气相二价阴离子C7H22-

采用HF, MP2, CCSD以及CISD方法,研究了二价阴离子C₇H₂^2-和C₇H₃^2-及其一价阴离子的几何结构振动频率。在理论上,我们得到了C₇H₂^2-和C₇H₃^2-能量最低结构分别为：C₂C(H₂)C₄^2-和C₂CHCHCHC₂^2-,而且均没有虚频。计算这两种结构所有可能的碎片化通道,碎片化能表明这两结构都不易解离为两个一价阴离子碎片。但是这两结构的垂直电离能和绝热电离能表明C₂C(H₂)C₄^2-是稳定的,但C₂CHCHCHC₂^2-是不稳定的。     

类单晶硅结构Si(C≡C―C_6H_4―C≡C)_4新材料的力学与光学性质:第一性原理研究

基于单晶硅中Si的四面体成键特征及对其结构单元的替换修饰,我们设计了一种类单晶硅结构的新材料-C_(40)H_(16)Si_2。通过广泛的第一性原理计算,研究了这类材料的电子性质、力学性质和光学性质。计算结果表明,这种新材料具有好的热力学稳定性和机械稳定性。该材料的禁带宽度为3.32 eV,价带底和导带顶都位于Gamma点,是直接带隙宽禁带半导体材料。该材料的维氏硬度和密度非常小,不到单晶硅的十分之一,是一类低密度的柔性多孔材料。此外,该材料在紫外光区有强的吸收,有望应用于蓝绿光发光二极管。     

从头算VB-MP2组合方法

提出了一种经济实用的价键计算方法VB-MP2方法.将体系的电子分为非活性电子和活性电子,应用MP2方法计算非活性电子的相关能效应,用VB方法处理活性电子.测试计算表明,该方法保持了价键方法的特点,且计算结果比传统的使用芯-价分离技术的价键方法有较大的改善.     

卤素氮氧化物的结构和光谱性质的理论研究

通过DFT/B3LYP计算,优化了卤素氮氧化物XNO_2及其异构体XONO（X = F,Cl ,Br,I）的平衡几何构型,预测了异构体的相对稳定性及其相互转化的活化能垒 。在B3LYP和QCISD（T）计算水平上,确定了X-NO_2键的解离能。应用与时间有关 的密度泛函理论（TD-B3LYP）计算了XNO_2低激发态的跃迁能,并讨论了这些激发 态与卤素氮氧化物光诱导解离过程的关联。     

羰基镍簇Ni(CO)n(n=1-4)的结构和Ni-CO键解离性质的密度泛函理论研究

在密度泛函理论框架下, 应用不同泛函计算了配合物Ni(CO)n(n=1~4)的平衡几何构型和振动频率. 考察了泛函和基组重叠误差对预测Ni—CO键解离能的影响. 计算结果表明, 用杂化泛函能得到与实验一致的优化几何构型和较合理的振动频率. 对Ni(CO)n(n=2~4)体系, 用“纯”泛函, 如BP86和BPW91, 可得到与CCSD(T)更符合、 并与实验值接近的解离能. 当解离产物出现单个金属原子或离子(如金属羰基配合物的完全解离)时, BSSE校正项的计算中应保持金属部分的电子结构一致. 只有考虑配体基组和不考虑配体基组两种情况下金属的电子构型与配合物中金属的构型一致时, 才能得到合理的BSSE校正, 从而预测合理的解离能.     

Differential diffusion quantum Monte Carlo method: determination of potential energy surfaces of molecules

A differential approach for self-optimizing diffusion Monte Carlo calculation was proposed in this paper, which is a new algorithm combining three techniques such as optimizing, diffusion and correlation sampling. This method can be used to directly compute the energy differential between two systems in the diffusion process, making the statistical error of calculation be reduced to order of 10-5 hartree, and recover about more than 80% of the correlation energy. We employed this approach to set up a potential energy surface of a molecule, used a "rigid move" model, and utilized Jacobi transformation to make energy calculation for two configurations of a molecule having good positive correlation. So, an accurate energy differential could be obtained, and the potential energy surface with good quality can be depicted. In calculation, a technique called "post-equilibrium remaining sample was set up firstly, which can save about 50% of computation expense. This novel algorithm was used to study the potenti