Computational chemistry methods based on MNDO and tools for improving their accuracy

CONTEXT: Semiempirical computational methods based on the MNDO approximation have a long history. Since its release in 1977, the original MNDO method has evolved steadily, passing through several forms (AM1, PM3, etc.) up to the present. Each new form represented an improvement, and it is anticipated that the evolution will continue for many more years. METHODS: A summary of the history of MNDO-based methods will be given, followed by a description of the mechanisms involved in developing some of the current methods. To assist in this, a set of files illustrating the steps and procedures involved is provided in Supporting Material; hopefully these will be useful in facilitating the development of future methods.