Interaction Region Indicator (IRI): A Very Simple Real Space Function Clearly Revealing Both Chemical Bonds and Weak Interactions

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.