About

MOPAC, the Molecular Orbital PACkage, is a computer program for calculating properties of atomistic systems such as molecules, crystals, and nanostructures using semiempirical quantum-mechanical models of thermochemistry. It primarily works through a simple command-line interface that takes an input file specifying a calculation and produces an output file summarizing the results of the calculation. However, it also has integration with multiple graphical user interfaces (GUIs) and an application programming interface (API) for a limited set of its functionality. The development of MOPAC began in 1981, its first public release was in 1983, and it has been regularly updated since then with ever-expanding functionality. The early versions of MOPAC were released into the public domain until it became commercial software in 1993, and it re-entered the public domain as an open-source software project in 2022.

The open-source version of MOPAC was developed by the Molecular Sciences Software Institute, which is funded by the National Science Foundation through grants ACI-1547580 and CHE-2136142.