Ab-Initio Identification of Atoms in Molecules

理论与计算化学项目由爱荷华州立大学K. Ruedenberg教授资助。这项研究将在未来两年进行，重点是发展一种新的量子化学理论，从而产生相应的新的计算化学代码。目标是在严格的从头算理论框架内定量确定分子中的原子，并根据这些原子之间的相互作用定量表达结合能。变形原子是通过它们在全价空间MCSCF分子轨道空间的最优波函数来确定的。定向准原子轨道，最大程度地参与特定的键和具有明确的电子居群，是通过密度矩阵分析确定的。结合能表现为原子内促进和原子间非键排斥、电子共享和电荷转移的贡献之和。通过确定无电荷转移的最佳分子MCSCF波函数来确定电荷转移引起的能量增益。共价能增益是从表示原子间电子迁移的构型推导出来的。分析适用于非平衡以及平衡几何，并将扩展到全价空间之外。尽管两个世纪以来，化学实验一直有效地以原子构成分子的概念为指导，但关于这一建立良好的经验模型的理论基础，主要未解决的问题一直持续到今天，而且它还没有在严格的量子力学计算的背景下被量化。这是因为分子中原子的概念实际上并不是基础理论中的一个基本概念。最近，Ruedenberg研究小组开发了一种新的分析方法，能够在严格的分子计算中识别原子亚基。在这种理论方法的帮助下，原子以及它们之间的相互作用将在分子中被定量地确定。期望所发展的方法能使理论计算的结果更直接地与实验员的直觉推理联系起来，从而使理论计算的结果对实际化学工作更有用。

The Theoretical and Computational Chemistry program is supporting Prof. K. Ruedenberg at Iowa State University. The research to be carried out over the next two years will focus on the development of a new quantum chemical theory that will result in corresponding new computational chemistry codes. The objective is the quantitative determination of atoms in molecules within the framework of rigorous ab-initio theory, and the quantitative expression of binding energies in terms of interactions between such atoms. The deformed atoms are determined through their optimal wave functions in terms of orbitals restricted to the full-valence-space MCSCF molecular orbital space. Oriented quasiatomic orbitals, maximally involved in specific bonds and with unambiguous electron populations, are determined through a density matrix analysis. The binding energy appears as the sum of contributions due to intra-atomic promotion and inter-atomic non-bonded repulsion, electron sharing and charge transfer. The energy gain due to charge transfer is identified by determining the optimal molecular MCSCF wave function without charge transfer. The covalent energy gain is derived from the configurations representing electron migrations between atoms. The analysis applies to non-equilibrium as well as to equilibrium geometries and will be extended beyond the full valence space. Although for two centuries chemical experimentation has been effectively guided by the concept of molecules being built from atoms, major unsolved problems have persisted to this day regarding the theoretical foundation of this well established empirical model and it has not yet been quantified within the context of rigorous quantum mechanical calculations. This is because the atoms-in-molecules concept is, in fact, not a fundamental one in basic theory. Recently, the Ruedenberg research group has developed a new analysis that is able to identify atomic sub-units in rigorous molecular calculations. With the help of this theoretical approach, atoms, as well as the interactions between them, will be quantitatively determined in molecules. The method to be developed is expected to make the results of theoretical computations more useful for practical chemical work by relating them more directly to the intuitive reasoning of experimentalists.