A Carbon Is a Carbon Is a Carbon: Orbital-Free Simulations of Hydrocarbon Chemistry without Resort to Atom Types

Semiclassical electrons (aka Lewis dots) have been a
mainstay of
chemists’ thinking about molecular structure, polarizability,
and reactivity for over a century. This utility has motivated the
development of a corresponding quantitative description. Here we devise
pairwise potentials that describe the behavior of valence electron
pairs in hydrocarbons, including those in single, double, bridge,
and bent bonds of linear, branched, and cyclic compounds, including
anionic and cationic states. Beyond predicting structures and energies,
the new subatomistic force field, dubbed LEWIS-B, efficiently simulates
carbocation addition to a double bond and cation migration to a neighboring
carbon. A crucial feature of the semiclassical electrons is variable
spread, a fourth degree of freedom in addition to three Cartesian
coordinates. In spontaneously adapting to different environments,
the spread provides a signature of electron stability, with more contracted
clouds where the electron interactions are favorable and expanded
clouds where electrons are less tightly held. In addition, the pair
potentials provide insight into the subtle trade-offs that govern
isomerizations and reactions.