Eric Hénon

Steric repulsion is one of the most frequently invoked concepts in chemistry, yet one of the least rigorously characterized: selectivity, conformational preferences, and reactivity are routinely rationalized by “steric clashes” depicted in a qualitative, often sketchy manner. The challenge is to convert its quantum-mechanical origin (the Pauli exclusion principle), a wavefunction-level constraint, into a real-space, chemically interpretable quantity revealing where exclusion operates and how much it costs. We introduce SELF (Steric Exclusion Localization Function), a new real-space descriptor that localizes and quantifies steric repulsion in three dimensions with atomic resolution, from a single routine quantum-mechanical calculation. SELF maps the local kinetic energy excess arising from same-spin exclusion between interacting electron distributions, delivering 3D isosurfaces, integrated repulsion scores (which correlate with EDA/SAPT exchange-repulsion energies), and per-atom decompositions. It applies equally to intermolecular contacts and, unlike most energy decomposition schemes, to intramolecular repulsion within a single structure, while naturally capturing the anisotropy of repulsion around atoms that distance-based steric models miss. Implemented in IGMPlot software, SELF adds a quantitative, energy-based measure of the steric repulsion that real-space indicators such as sign(λ₂)ρ identify but do not quantify energetically. Selected applications, from sterically controlled organocatalysis to π-hole interaction, will illustrate its scope. SELF bridges quantum theory and experimental practice, offering a practical tool for rational design, mechanistic analysis, and teaching.