MOPAC (OpenMOPAC)
The modern open-source continuation of MOPAC provides semiempirical molecular-orbital calculations for molecules and materials.
Where it fits
Fast screening, preliminary structures and teaching computational chemistry.
Limits and practical considerations
Parameter coverage and simplified Hamiltonians limit predictive accuracy; validate important results at higher levels.
Licensing and access
Modern OpenMOPAC is Apache-2.0 licensed and maintained by MolSSI; older commercial MOPAC licensing descriptions do not apply to this distribution.
Official documentation and project