Relationship of AMPAC and MOPAC

In 1985 MOPAC 3.0 and AMPAC 1.0 were submitted to QCPE for distribution. At that time, AMPAC differed from MOPAC in that it had the AM1 algorithm. Additionally, changes in some MNDO parameters in AMPAC made AMPAC results incompatable with MOPAC Versions 1-3. Subsequent versions of MOPAC, in addition to being more highly debugged than Version 3.0, also had the AM1 method. Such versions were compatible with AMPAC and with versions 1-3 of MOPAC.

In order to avoid confusion, all versions of MOPAC after 3.0 include journal references so that the user knows unambiguously which parameter sets were used in any given job.

Since 1985 AMPAC and MOPAC have evolved along different lines. In MOPAC I have endeavoured to provide a highly robust program, one with only a few new features, but which is easily portable and which can be relied upon to give precise, if not very exciting, answers. At Austin, the functionality of AMPAC has been enhanced by the research work of Prof. Dewar's group. The new AMPAC 2.1 thus has functionalities not present in MOPAC. In publications, users should cite not only the program name but also the version number.

Commercial concerns have optimized both MOPAC and AMPAC for use on supercomputers. The quality of optimization and the degree to which the parent algorithm has been preserved differs between MOPAC and AMPAC and also between some machine specific versions. Different users may prefer one program to the other, based on considerations such as speed. Some modifications of AMPAC run faster than some modifications of MOPAC, and vice versa, but if these are modified versions of MOPAC 3.0 or AMPAC 1.0, they represent the programming prowess of the companies doing the conversion, and not any intrinsic difference between the two programs.

Testing of these large algorithms is difficult, and several times users have reported bugs in MOPAC or AMPAC which were introduced after they were supplied by QCPE.