AC/DC Analysis: Broad and Comprehensive Approach to Analyze Infrared Intensities at the Atomic Level

We present a complete theoretical protocol to partition infrared intensities into terms owing to individual atoms by two different but related approaches: the atomic contributions (ACs) show how the entire molecular vibrational motion affects the electronic structure of a single atom and the total infrared intensity. On the other hand, the dynamic contributions (DCs) show how the displacement of a single atom alters the electronic structure of the entire molecule and the total intensity. The two analyses are complementary ways of partitioning the same total intensity and conserve most of the features of the total intensity itself. Combined, they are called the AC/DC analysis. These can be further partitioned following the CCTDP (or CCT) models according to the population analysis chosen by the researcher. The main conceptual features of the equations are highlighted, and representative numerical results are shown to support the interpretation of the equations. The results are invariant to rotation and translation and can readily be extended to molecules of any size, shape, or symmetry. Although the AC/DC analysis requires the choice of a charge model, all charge models that correctly reproduce the total molecular dipole moment can be used. A fully automated protocol managed by the Placzek program is made available, free of charge and with input examples.本研究提出一套完整的理论框架，可通过两种不同却相关的方法，将红外强度分解为各单个原子的贡献项。原子贡献（Atomic Contributions, ACs）用于阐明整个分子的振动运动如何影响单个原子的电子结构与总红外强度；与之相对，动态贡献（Dynamic Contributions, DCs）则用于说明单个原子的位移如何改变整个分子的电子结构与总红外强度。这两种分析方法互为补充，均针对同一总红外强度进行分解，且保留了总红外强度的绝大多数固有特征，二者结合即称为AC/DC分析。该分析框架可根据研究者选择的布居分析方法，进一步依据CCTDP（或CCT）模型进行细分。本研究着重阐明了相关方程的核心概念特征，并展示了代表性数值结果以佐证对这些方程的解读。所得结果不受旋转与平移操作的影响，且可便捷推广至任意尺寸、形状或对称性的分子体系。尽管AC/DC分析需要选取电荷模型，但所有能够准确重现分子总偶极矩的电荷模型均可适用。本研究提供了一套由Placzek程序管理的全自动流程，可免费获取并附带输入示例。