MOPAC and semiempirical models: past and future

Semiempirical electronic structure originated as a stopgap from a time when first-principles electronic structure calculations

were intractable, and its utility has endured to the present day as a unique balance of cost, accuracy, and transferability

between cheaper-but-less-transferrable interatomic potentials and pricier-but-more-accurate first-principles methods.

We particularly highlight the Molecular Orbital PACkage (MOPAC), a venerable piece of semiempirical thermochemistry software

that has been specialized in recent years to enable large-scale simulations of enzymatic reactions in proteins and is now open source.

We also present a new technical vision for semiempirical electronic structure that unifies its fragmented methodologies

and might eventually serve the diverse needs of the modern userbase of atomistic simulation software.