molecules Article Spin Density Topology Giovanna Bruno 1 , Giovanni Macetti 2 , Leonardo Lo Presti 1 and Carlo Gatti 3,4,* 1 Dipartimento di Chimica, Università degli Studi di Milano, via Golgi 19, 20133 Milano, Italy;
[email protected] (G.B.);
[email protected] (L.L.P.) 2 Laboratoire de Physique et Chimie Théoriques (LPCT), Université de Lorraine & CNRS, 1 Boulevard Arago, F–57078 Metz, France;
[email protected] 3 CNR–SCITEC, Istituto di Scienze e Tecnologie Chimiche sezione di via Golgi, c/o Università degli Studi di Milano, via Golgi 19, 20133 Milano, Italy 4 Istituto Lombardo, Accademia di Scienze e Lettere, via Brera 28, 20100 Milano, Italy * Correspondence:
[email protected] Academic Editor: Ángel Martín Pendás Received: 30 June 2020; Accepted: 1 August 2020; Published: 2 August 2020 Abstract: Despite its role in spin density functional theory and it being the basic observable for describing and understanding magnetic phenomena, few studies have appeared on the electron spin density subtleties thus far. A systematic full topological analysis of this function is lacking, seemingly in contrast to the blossoming in the last 20 years of many studies on the topological features of other scalar fields of chemical interest. We aim to fill this gap by unveiling the kind of information hidden in the spin density distribution that only its topology can disclose. The significance of the spin density critical points, the 18 different ways in which they can be realized and the peculiar topological constraints on their number and kind, arising from the presence of positive and negative spin density regions, is addressed.