Examples of the chronological progression from “qualitative soft” to “quantitative hard”: for steric effects and for electronic effects.

	Steric effects	Electronic effects
Qualitative soft	From the 1870s observation of steric effects in the alkylation of amines51,52	Robinson's198,199 and Ingold's58 electronic theories
Quantitative soft	Taft's linear free energy relationships76 and force field calculations of steric effects75	Hammett's equations61,62
Qualitative hard	VSEPR (valence shell electron pair repulsion)200,201	Through-bond effects by eHT202–204
Quantitative hard	Numerous quantitative evaluations based on quantum chemical calculations that have their beginnings in the 1970s and early 1980s.75,84,205,206	Various flavors of energy decomposition analysis158,167