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Energy decomposition analysis (EDA) has become an important tool to relate electronic structure calculations to physically meaningful contributions. The second generation of the absolutely localized molecular orbitals (ALMO)-EDA accounts for polarization with a well- defined basis set limit using truncated virtual orbitals, namely fragment electric-field response functions (FERF). In this work, we introduce a hessian-free uncoupled FERF (uFERF) al- ternative that has very similar accuracy and is 8-10 times faster to evaluate. Furthermore, we investigate the use of monopole uFERFs (response to scaled nuclear charges) for inter-molecular interactions and establish their role in strong ion-neutral interactions. 

The chemical bond is the cornerstone of chemistry, providing a conceptual framework to understand and predict the behavior of molecules in complex systems. However, the fundamental origin of chemical bonding remains controversial, and has been responsible for fierce debate over the past century. Here we present a unified theory of bonding, using a separation of electron delocalization effects from orbital relaxation to identify three mechanisms -- node-induced confinement (typically associated with Pauli repulsion, though more general), orbital contraction and polarization -- that each modulate kinetic energy during bond formation. Through analysis of a series of archetypal bonds, we show that an exquisite balance of energy-lowering delocalizing and localizing effects are dictated simply by atomic electron configurations, nodal structure and electronegativities. The utility of this unified bonding theory is demonstrated by its application to explain observed trends in bond strengths throughout the periodic table, including main group and transition metal elements.