Systematic treatment of spin-reactivity indicators in conceptual density functional theory

The spin-resolved conceptual density functional theory is presented, emphasizing the finite-difference approximations for spin-reactivity indicators and especially the spin-hardness. We derive simple approximations for these spin-reactivity indicators in terms of Kohn–Sham orbital energies and discuss their accuracy and validity. We also show that the second derivative of the energy with respect to the spin-abundance, NS = ½MS, is nonnegative, contradicting the generally negative values obtained for one of the most popular formulas used to approximate this quantity. A new approximation for the spin-hardness is proposed and assessed based on information for spin-philicities and singlet–triplet gaps. One conclusion of our work is that the tendency in the literature to mix linear models (e.g., one-sided derivatives for chemical potentials and Fukui functions) with quadratic models (e.g., two-sided derivatives for hardnesses and dual descriptors) not only is theoretically unsound, but often leads to chemically absurd results.