Bonds, lone pairs, and shells probed by means of on-top dynamical correlations

The electron localization function (ELF) by Becke and Edgecombe [A.D. Becke, K.E. Edgecombe,
J. Chem. Phys. 92, 5397 (1990)] is routinely adopted as a descriptor of atomic shells and covalent
bonds. Since the ELF and its related quantities find useful exploitation also in the construction of modern
density functionals, the interest in complementing the ELF is linked to both the quests of improving
electronic structure descriptors and density functional approximations. The ELF uses information which
is available by considering parallel-spin electron pairs in single-reference many-body states. In this work,
we complement this construction with information obtained by considering antiparallel-spin pairs whose
short-range correlations are modeled by a density functional approximation. As a result, the approach
requires only a contained computational effort. Applications to a variety of systems show that, in this way,
we gain a spatial description of the bond in H2 (which is not available with the ELF) together with some
trends not optimally captured by the ELF in other prototypical situations.