Martin P. Head-Gordon

Today, computational quantum chemistry calculations can provide near-quantitative answers to questions about the strength of covalent interactions, like chemical bonds, and non-covalent interactions like hydrogen bonds, with only modest compute requirements. This is a triumph of the quantum chemistry community, through development of state-of-the-art density functional theory (DFT) over the past 60 years. Yet the results of those calculations are like numerical experiments. They tell you the value of the quantity: for instance that the water-water hydrogen bond is 20 kJ/mol, or the H-H bond in hydrogen is over 400 kJ/mol. They do not tell you the driving force that causes the interaction. To fill this gap, it is desirable to have quantitative ways to associate contributions to binding with physical and chemical driving forces. This is the task of energy decomposition analysis (EDA), and although EDAs cannot be considered completely mature, they are increasingly used for applications such as understanding non-covalent interactions, chemical bonds, and even aspects of chemical catalysis and reactivity. I will describe key elements that define the Absolutely Localized MO (ALMO) EDA, which attempts to decompose an interaction energy such as a bond or hydrogen bond into underlying physical driving forces, such as Pauli repulsions, spin-pairing, permanent and induced electrostatics, dispersion, and charge-transfer. I will discuss what this EDA reveals about the old yet and yet not fully resolved question of the driving force behind conventional covalent bonding, as well as some unconventional bonds.