SINGLET-TRIPLET GAPS AND SPIN POTENTIALS

The relation between vertical and adiabatic singlet−triplet gaps and the generalized spin-dependent global response coefficients is presented. Local, semilocal, and hybrid density functional calculations of these quantities for the halocarbenes CXY, where X and Y are H, F, Cl, Br, and I, show that the vertical gaps are better reproduced at the local level. Hybrid functionals overestimate them considerably. For the whole series of halocarbenes considered, except CHI, calculations predict the singlet 1A1 as the ground state. The only exception found is for the hybrid calculation of CHI that predicts 3B1 as the ground state for this halocarbene. For all functionals, there is a linear relation between the vertical singlet−triplet energy gap and the spin potential (the first derivative of the total energy with respect to the number of unpaired electrons), and the inclusion of the spin hardness (the second derivative of the total energy with respect to the number of unpaired electrons) improves this relationshi...