External fields in conceptual density functional theory

The necessity of the recent incorporation of new external variables in the context of conceptual DFT (CDFT) is discussed based on the ever‐increasing portfolio of experimental reaction conditions in the endeavor of experimentalists to synthesize new molecules with unprecedented properties. Electric and magnetic fields (ε and B), mechanical forces (F), and confinement are proposed as valuable new variables, extending conventional CDFT and its associated response functions. A finite field approach is used to calculate the evolution of both global and local descriptors in a selected series of atomic and molecular applications, and from it derive new response function involving, with one exception, the first derivative to the field considered. The electric field results, displaying, for example, a case of a field‐induced enantioselectivity in the Fukui function, may be instrumental in the recent upsurge of chemistry in oriented external electric fields. The study of atomic electronegativity and hardness in magnetic fields displays a piecewise behavior, associated to configurational jumps upon increasing field strength and reveals an overall compression of their ranges for stronger fields, which may be guiding upon investigating chemistry in extremely high fields like in white dwarfs. The evolution of the electronegativity and hardness of diatomics under mechanical force can elegantly be traced back to differences in their equilibrium distance in the neutral, cationic, and anionic state. The well‐known reduction of the polarizability under confinement can be seen as a fore‐runner of the increasing hardness of atoms under pressure, presently under investigation. Periodicity showing up in a spontaneous way in the variety of properties is a leitmotiv in this study, as well as the interconnections/analogies between the different response functions. The necessity of the recent incorporation of new external variables in the context of conceptual density functional theory (CDFT) is discussed based on the ever‐increasing portfolio of experimental reaction conditions in the endeavor of experimentalists to synthesize new molecules with unprecedented properties. Reactivity descriptors from CDFT, such as the electronegativity χ, the chemical hardness η, the electrophilicity ω, and the Fukui function f(r), are used to get insight into the chemistry under these new conditions, such as the presence of external electric (ε) and magnetic (B) fields, mechanical forces (F) and co