LEWIS is a subatomistic force field based on semiclassical modeling of valence electron pairs: valence electron pairs and kernels (comprising nuclei with their core electrons) are treated as independently mobile particles that influence each other via pair interactions that reflect the underlying wave properties of the electrons. This approach has the advantage of providing a natural and efficient description of polarizability and reactivity. In recent work, the transferability of the interaction potentials among molecules containing 2p elements has been improved by allowing each electron pair a variable cloud size, (with corresponding quantum kinetic energy), in addition to its three spatial coordinates. A remaining difficulty has been the prediction of dihedral angles and associated dipole moments in molecules such as H2O2 and N2H4. In this context we consider several different forms for the exchange contributions to the energy and identify the feature that is essential for describing skewed conformations, specifically that the curvature of the exchange energy at interatomic distances needs to be negative so that electron pairs at either end of a bond prefer an eclipsed conformation, in opposition to the preference of kernel-kernel repulsions for staggered conformations.

Developments in the LEWIS force field for semi-classical valence electron pairs: Origins of skewed dihedral angles in small molecules

ACS Fall 2021

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