Graphically revealing interaction regions in a chemical system enables chemists to notice the areas at a glance where significant interactions have formed, it is very helpful in studying chemical bonds, intermolecular and intramolecular interactions. Reduced density gradient (RDG) has already been widely employed in literatures to visually exhibit weak interaction regions, in fact it also has the ability of revealing chemical bonding regions. Unfortunately, RDG cannot clearly show both types of the interactions at the same time. In this paper, we propose a new real space function named interaction region indicator (IRI), which is a slight modification on RDG. We found IRI can reveal chemical bonding and weak interaction regions equally well, this brings great convenience in the study of various chemical systems as well as chemical reactions. It is noteworthy that IRI has simpler definition, lower computational cost and better graphical effect than the density overlap regions indicator (DORI), which has similar purpose to IRI. In this article IRI is also compared with atom-in-molecules (AIM) topology analysis of electron density, we demonstrated that IRI has the ability to reveal additional interactions to provide chemists a more complete picture. In addition, we put forward a variant of IRI named IRI-pi, which is dedicated to reveal interactions of pi electrons. It is found that IRI-pi can not only distinguish type of pi interactions but can also exhibit pi-interaction strength. IRI and IRI-pi have been efficiently implemented in our freely available Multiwfn wavefunction analysis code, it is expected that they will become new useful members of computational chemists' toolbox in studying chemical problems.

中文翻译：

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相互作用区域指示器（IRI）：一个非常简单的实空间函数，清楚地揭示了化学键和弱相互作用

以图形方式显示化学系统中的相互作用区域，使化学家能够一眼注意到形成显着相互作用的区域，这对于研究化学键，分子间和分子内相互作用非常有帮助。降低密度梯度（RDG）已经在文献中被广泛采用，以在视觉上显示弱的相互作用区域，实际上，它还具有揭示化学键合区域的能力。不幸的是，RDG无法同时清楚地显示两种类型的交互。在本文中，我们提出了一个新的称为交互区域指示器（IRI）的实空间函数，它是对RDG的略微修改。我们发现IRI可以很好地揭示化学键和弱相互作用区域，这为研究各种化学系统以及化学反应提供了极大的便利。值得注意的是，与密度重叠区域指示符（DORI）相比，IRI具有更简单的定义，更低的计算成本和更好的图形效果，其目的与IRI相似。在本文中，还将IRI与电子密度的分子内原子（AIM）拓扑分析进行了比较，我们证明了IRI具有揭示其他相互作用的能力，从而为化学家提供了更完整的图片。另外，我们提出了一个IRI的变体，称为IRI-pi，该变体专门用于揭示π电子的相互作用。发现IRI-pi不仅可以区分pi相互作用的类型，而且可以表现出pi相互作用的强度。IRI和IRI-pi已在我们免费提供的Multiwfn波函数分析代码中得到有效实施，预计它们将成为计算化学家的新有用成员。