Bonding Analysis of Ab Initio Electronic Wave Functions in Molecules

爱荷华州立大学的Klaus Ruedenberg和Mark Gordon获得了化学系化学理论、模型和计算方法计划颁发的奖项的支持，以继续开发一种严格且广泛适用的方法来了解分子和分子系统中发生的化学键。这种成键分析定量地识别和分类了在分子中原子之间产生键的不同类型的物理相互作用。到目前为止，对化学中的这一基本现象还没有一个严格的理解。虽然‘键’的发现可以追溯到200年前，但直到100年后，氢分子中最简单的化学键才能在量子力学的基础上进行定量计算。由于最近计算机的发展，许多分子的能量现在可以相当准确地计算出来。然而，各种化学键的物理起源仍然难以捉摸。例如，只有猜测为什么一氧化碳分子比氮分子有更强的键，尽管这些简单的分子具有非常相似的电子结构。在目前的分析中，通过从理论计算中提取这些信息，建立了对相关和熟悉的物理相互作用中键能的严格定量理解。与目前的定性合理化相比，该分析为实习化学家在理解和预测键合模式方面提供了更可靠的直观使用指南。该分析也为化学结合的物理起源教学提供了坚实的理论基础。该项目的目标是增强严格的量子力学计算对实用化学的影响。成键分析包含在GAMESS(通用原子和分子结构系统)程序套件中，从分子电子从头算波函数出发，推导出与自由原子的原子轨道有很大重叠的轨道，同时与分子中的成键模式相适应。波函数是用这些准原子轨道表示的，因此它是三项之和。第一项体现了处于价态的并列原子，它们之间没有电子迁移，因此没有共价键相互作用。第二项体现了中性原子之间的电子共享，因此是共价键。第三项体现了原子之间的电荷转移。从波函数的这一解中，能量的分解可以用代表以下相互作用的五个相加贡献来获得：原子价态的形成、准经典库仑相互作用、干涉相互作用、共享穿透相互作用和电荷转移相互作用。这些相互作用进一步分解为各种轨道的贡献。在这些分辨率的基础上，对连接特定分子中不同原子的键进行了表征和比较。

Klaus Ruedenberg and Mark Gordon of Iowa State University are supported by an award from the Chemical Theory, Models and Computational Methods Program in the Chemistry Division to continue the development of a rigorous and widely applicable method for understanding the chemical bonding that occurs in molecules and molecular systems. This bonding analysis quantitatively identifies and sorts out the different types of physical interactions that act together in creating bonds between atoms in molecules. As yet, a rigorous understanding does not exist for this fundamental phenomenon in chemistry. While the discovery of 'bonds' goes back two hundred years, it was only a hundred years later that that the simplest chemical bond, in the hydrogen molecule, could be quantitatively calculated on the basis of quantum mechanics. As a result of the recent development of computers, energies of many molecules can now be calculated quite accurately. However, the physical origins of the vast diversity of chemical bonds are still elusive. For instance, only speculations exist on why the carbon monoxide molecule has a stronger bond than the nitrogen molecule even though these simple molecules have very similar electronic structures. In the present analysis, a rigorous quantitative understanding of bond energies in terms of relevant and familiar physical interactions is developed by extracting this information from theoretical calculations. The analysis provides a more reliable guide for intuitive use by practicing chemists in understanding and anticipating bonding patterns than current qualitative rationalizations. The analysis is also provides sound material for teaching the physical origin of chemical binding based on a solid theoretical basis. The goal of the project is to enhance the impact of rigorous quantum mechanical calculations on practical chemistry. The bonding analysis is included in the freely available and widely used GAMESS (General Atomic and Molecular Electronic Structure System) program suite.From the molecular electronic ab initio wave function, orbitals are deduced that have very large overlap with atomic orbitals of the free atoms and, at the same time, are adapted to the bonding pattern in the molecule. The wave function is expressed in terms of these quasi-atomic orbitals and it, thereby, becomes a sum of three terms. The first term embodies the juxtaposed atoms in their valence states without electron migration between them and, hence, without covalent bonding interactions. The second term embodies electron sharing, and hence, covalent bonding between the neutral atoms. The third term embodies charge transfers between the atoms. From this resolution of the wave function, a decomposition of the energy is obtained in terms of five additive contributions that represent the following interactions: formation of the atomic valence states, quasi-classical Coulombic interactions, interference interactions, sharing-penetration interactions, and charge transfer interactions. These interactions are furthermore resolved into contributions of the various orbitals. On the basis of these resolutions, the bonds that link various atoms in specific molecules are characterized and compared.