Confronting modern valence bond theory with momentum-space quantum similarity and with pair density analysis

The ever increasing level of sophistication of modern quantum chemical computations tends to make it more and more difficult to find direct links with the various classical models still employed by most chemists to visualize and to interpret molecular electronic structure. This has led to the development of a wide variety of schemes for the direct analysis of total wavefunctions and, especially, of total electron densities. Running against the general trend is the renaissance of valence bond (VB) theory, which aims to provide numerical accuracy with models that are relatively simple to interpret directly. This is especially true of modern developments such as spin-coupled theory [1–3], which combine useful accuracy with conceptually simple descriptions of the behaviour of correlated electrons. In the present work, we confront descriptions inferred directly from spin-coupled calculations with momentum-space quantum similarity indices for electron densities and with pair density analyses of total wavefunctions. By way of examples, we consider two gas-phase pericyclic reactions, namely the parent Diels-Alder process and the disrotatory ring-opening of cyclohexadiene. Additionally, we use the PF4CH3 molecule as an example to examine the nature of the bonding to hypercoordinate main group atoms.