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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. 

In charged water microdroplets, which occur in nature or in the lab upon ultrasonication or in electrospray processes, the thermodynamics for reactive chemistry can be dramatically altered relative to the bulk phase. Here, we provide a theoretical basis for the observation of accelerated chemistry by simulating water droplets of increasing charge imbalance to create redox agents such as hydroxyl and hydrogen radicals and solvated electrons. We compute the hydration enthalpy of OH⁻and H⁺that controls the electron transfer process, and the corresponding changes in vertical ionization energy and vertical electron affinity of the ions, to create OH^•and H^•reactive species. We find that at ~ 20 − 50% of the Rayleigh limit of droplet charge the hydration enthalpy of both OH⁻and H⁺have decreased by >50 kcal/mol such that electron transfer becomes thermodynamically favorable, in correspondence with the more favorable vertical electron affinity of H⁺and the lowered vertical ionization energy of OH⁻. We provide scaling arguments that show that the nanoscale calculations and conclusions extend to the experimental microdroplet length scale. The relevance of the droplet charge for chemical reactivity is illustrated for the formation of H₂O₂, and has clear implications for other redox reactions observed to occur with enhanced rates in microdroplets. 

Identification of the breaking point for the chemical bond is essential for our understanding of chemical reactivity. 

Water is often the testing ground for new advanced force fields. While advanced functional forms for intermolecular interactions have been integral to the development of accurate water models, less attention has been paid to a transferable model for the intramolecular valence terms. In this work, we present a 1-body energy and dipole moment surface model, named 1B-UCB, that is simple yet accurate, and which can be feasibly adapted for both standard and advanced potentials. 1B-UCB for water is comparable in accuracy to those with much more complex functional forms despite having drastically fewer parameters. The parameterization protocol has been implemented as part of Q-Force automated workflow and requires only a QM Hessian calculation as reference data, hence allowing it to be easily extended to a variety of molecular systems beyond water, which we demonstrate on a selection of small molecules with different symmetries. 

In this article, we introduce the occupied-virtual orbitals for chemical valence (OVOCV). The OVOCVs can replace or complement the closely related idea of the natural orbitals for chemical valence (NOCV). The input is a difference density matrix connecting any initial single determinant to any final determinant, at a given molecular geometry, and a given one-particle basis. This arises in problems such as orbital rearrangement or charge-transfer in energy decomposition analysis. The OVOCVs block-diagonalize the density difference operator into 2 × 2 blocks which are spanned by one level that is filled in the initial state (the occupied OVOCV) and one which is empty (the virtual OVOCV). By contrast, the NOCVs fully diagonalize the density difference matrix, and therefore are orbitals with mixed occupied-virtual character. Use of the OVOCVs makes it much easier to identify the donor and acceptor orbitals. We also introduce two different types of energy decomposition analysis (EDA) methods with the OVOCVs, and most importantly, a charge decomposition analysis (CDA) method that fixes the unreasonably large charge transfer amount obtained directly from NOCV analysis. 

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.