The Molecular Orbital PACkage (MOPAC) began its development in 1981 as a consolidation of the semiempirical thermochemistry model and software development activities of the Dewar group at UT Austin in the 1970’s into a unified, user-friendly software package. Over time, it has steadily expanded its simulation capabilities from small gas-phase organic molecules to most of the periodic table, molecules in solution, solid-state systems, and large-scale simulations using novel solver algorithms.
MOPAC has been developed as commercial software for the last 30 years, and MolSSI has facilitated
its transition to an open-source software project that provides it with the opportunity to be maintained
and further developed by a community of open-source software developers.
The central feature of MOPAC are its semiempirical models, which have the basic structure of Hartree-Fock calculations in a minimal basis with simplified and parameterized Hamiltonian matrix elements. These models are parameterized to fit a variety of high-quality experimental and theoretical reference data including heats of formation, geometric data, ionization potentials, electron affinities, and dipole moments.
Historically, the most popular semiempirical thermochemistry model was Austin Model 1 (AM1), and MOPAC served as a software platform for development of the PMx models (x=3,6,7) that have succeeded and refined the AM1 model. In the last decade, there has been an increasing amount of integration with force-field-like corrections to add dispersion effects and improve the description of hydrogen bonds.
Perhaps the single most important feature added to MOPAC was the COnductor-like Screening MOdel (COSMO) introduced in the mid-1990’s. COSMO is a popular implicit solvation model that replaces the ensemble of solvent molecules with a dielectric continuum, and the solute molecule being simulated is embedded in a cavity within this continuum. Such models enable a treatment of solvation effects in small, deterministic calculations that avoid sampling from the statistical mechanics of the solvent. MOPAC contains the original COSMO implementation by Andreas Klamt.
The most unique feature of MOPAC is its reduced-scaling solver, MOZYME. As its name suggests, MOZYME was developed to model enzymes, which host chemical reactions that are not easy to describe with force fields inside large protein structures that are expensive to simulate with first-principles quantum chemistry methods. MOZYME thus offers a unique balance between cost and accuracy that is not available in any other atomistic simulation software. A limitation of MOZYME is that it needs to identify a Lewis structure of chemical bonds to initialize its calculations. While this excludes some inorganic chemistry and materials science applications, MOZYME can still be applied beyond proteins and enzymes to systems such as organic polymers, carbon nanostructures, and metal-organic frameworks.
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