EMR and optical spectroscopy are important techniques enabling characterization of transition 3d/4f ions in coordination compounds, which often exhibit low symmetry sites. Low symmetry aspects are largely unexplored as yet due to lack of consistent methodology and guidelines, which has often led to their neglect or employing orthorhombic or higher symmetry approximations. To alleviate these problems, we provide comprehensive elucidation of low symmetry aspects in EMR, which are also relevant, to a great extent, for optical spectroscopy. Based on extensive survey of EMR-related monographs, reviews, selected papers and computer programs, definitions of the crucial notions underlying EMR studies of 3d/4f ions ions at low symmetry sites are systematized and their interrelationships clarified. Peculiar issues at the interface between theory, computations, and experiment, which affect proper interpretation of EMR data for low site symmetry systems, are discussed. Outline of the shortcomings, which may lead to employing deficient procedures in analysis of EMR data, is provided to help avoiding such deficiencies. To ensure comprehensive and up-to-date coverage, the survey of reviews and selected papers is supplemented by database of EMR studies of low site symmetry systems listing source references provided in Supplementary Materials. This review provides grounds for specialized reviews of low symmetry aspects occurring in: (i) studies of molecular nanomagnets containing transition ions, and (ii) density functional theory/ab initio modeling of zero-field splitting and crystal-field parameters.

中文翻译：

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电子磁共振（EMR）和过渡离子配位化合物光谱中的低对称性：关键概念和问题、文献调查和数据库

EMR 和光谱学是能够表征配位化合物中的过渡 3d/4f 离子的重要技术，这些离子通常表现出低对称性位点。由于缺乏一致的方法和指南，低对称性方面在很大程度上尚未被探索，这常常导致它们被忽视或采用正交或更高的对称性近似。为了缓解这些问题，我们对 EMR 中的低对称性方面进行了全面的阐述，这在很大程度上也与光谱学相关。基于对 EMR 相关专着、评论、精选论文和计算机程序的广泛调查，系统化了低对称位点 3d/4f 离子 EMR 研究的关键概念的定义，并阐明了它们之间的相互关系。讨论了理论、计算和实验之间的特殊问题，这些问题影响了低位点对称系统的 EMR 数据的正确解释。概述了可能导致在 EMR 数据分析中采用有缺陷的程序的缺陷，以帮助避免此类缺陷。为了确保全面和最新的覆盖范围，对评论和选定论文的调查由低位点对称系统的 EMR 研究数据库进行了补充，其中列出了补充材料中提供的源参考文献。本综述为对低对称性方面的专门综述提供了基础：(i) 含有过渡离子的分子纳米磁体的研究，以及 (ii) 零场分裂和晶体场参数的密度泛函理论/从头开始建模。