MOPAC is a general-purpose, semiempirical molecular orbital program used 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 of vibrational spectra, thermodynamic quantities, isotopic substitution effects, and force constants. MOPAC can calculate molecular and localized orbitals, excited states, chemical bond indices, charges, intrinsic and dynamic reaction coordinates, and transition-state location and optimization routines. Computational chemistry has evolved significantly over the past two decades, with three main branches—molecular mechanics, semiempirical, and ab initio—each having its own niche. Semiempirical methods, such as MNDO, MINDO/3, and AM1, are popular but not particularly fast or rigorous. Despite not being specialized, they are highly versatile. MOPAC is a single program incorporating many separate functions, developed by various groups before its creation. It was written in pure FORTRAN-77 for a VAX computer but has since been adapted to run on many machines, including supercomputers and microcomputers. MOPAC is in the public domain, with source code and documentation available from the Quantum Chemistry Program Exchange. It is updated annually to keep users current, with six versions published so far, adapted to 15–20 computer types. Users are advised to obtain MOPAC from QCPE to avoid confusion from modifications. MOPAC is described as 'impressively easy to use' and 'contains options for nearly all the applications where our procedures have been found to be useful'.MOPAC is a general-purpose, semiempirical molecular orbital program used 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 of vibrational spectra, thermodynamic quantities, isotopic substitution effects, and force constants. MOPAC can calculate molecular and localized orbitals, excited states, chemical bond indices, charges, intrinsic and dynamic reaction coordinates, and transition-state location and optimization routines. Computational chemistry has evolved significantly over the past two decades, with three main branches—molecular mechanics, semiempirical, and ab initio—each having its own niche. Semiempirical methods, such as MNDO, MINDO/3, and AM1, are popular but not particularly fast or rigorous. Despite not being specialized, they are highly versatile. MOPAC is a single program incorporating many separate functions, developed by various groups before its creation. It was written in pure FORTRAN-77 for a VAX computer but has since been adapted to run on many machines, including supercomputers and microcomputers. MOPAC is in the public domain, with source code and documentation available from the Quantum Chemistry Program Exchange. It is updated annually to keep users current, with six versions published so far, adapted to 15–20 computer types. Users are advised to obtain MOPAC from QCPE to avoid confusion from modifications. MOPAC is described as 'impressively easy to use' and 'contains options for nearly all the applications where our procedures have been found to be useful'.