Albert John Coleman – Teacher, researcher, and father of modern reduced density matrix theory

We introduce the localized pair model of electronic structure analysis and propose the two-electron reduced density matrix as an important interpretive tool in chemistry. Interelectronic probability distributions in position and momentum space are calculated for individual localized molecular orbitals corresponding to intuitive chemical features such as lone pairs and chemical bonds. It is demonstrated that these may be interpreted as the distribution of electrons within a chemical bond or lone pair and we refer to this model as the localized pair model of electronic structure analysis. Specifically, the Hartree–Fock level of theory is employed in conjunction with a completely uncontracted 6-311G (d,p) basis set to construct our localized orbitals. Spherically averaged position and momentum intracules are calculated for each orbital and we present results for orbitals of p-block hydrides, saturated main group compounds, fluorinated species, N → B dative structures, and small cyclic molecules. We find that our analysis generally agrees quite well with intuitive predictions based on bond lengths and electronegativities of the bonded atoms. However the trends in the data cannot be predicted using the bond length or electronegativity alone, which demonstrates the unique features of this model.