MOPAC is a semiempirical molecular orbital program designed for studying chemical reactions involving molecules, ions, and linear polymers. It implements semiempirical Hamiltonians such as MNDO, AM1, MINDO/3, and MNDO-PM3, and integrates calculations for vibrational spectra, thermodynamic quantities, isotopic substitution effects, and force constants. The program supports a wide range of elements and can compute molecular and localized orbitals, excited states, chemical bond indices, charges, and reaction coordinates. MOPAC is known for its versatility and ease of use, despite being slower than molecular mechanics methods and less rigorous than ab initio methods. The program has been developed over two decades, incorporating contributions from various research groups, and is available in the public domain. It has been updated annually to maintain currency and compatibility, and users are advised to obtain the latest version from the Quantum Chemistry Program Exchange to avoid confusion.MOPAC is a semiempirical molecular orbital program designed for studying chemical reactions involving molecules, ions, and linear polymers. It implements semiempirical Hamiltonians such as MNDO, AM1, MINDO/3, and MNDO-PM3, and integrates calculations for vibrational spectra, thermodynamic quantities, isotopic substitution effects, and force constants. The program supports a wide range of elements and can compute molecular and localized orbitals, excited states, chemical bond indices, charges, and reaction coordinates. MOPAC is known for its versatility and ease of use, despite being slower than molecular mechanics methods and less rigorous than ab initio methods. The program has been developed over two decades, incorporating contributions from various research groups, and is available in the public domain. It has been updated annually to maintain currency and compatibility, and users are advised to obtain the latest version from the Quantum Chemistry Program Exchange to avoid confusion.