The default geometry optimizer in MOPAC uses Baker's EigenFollowing method. If this is *not* wanted, for example, if there is a need to reduce memory demands, then the Broyden Fletcher Goldfarb Shanno method can be used.

The most common use of MOPAC is for geometry optimization. This involves starting with an approximation to the desired geometry and, by calculating the forces acting on the system, changing the geometry so as to lower the total energy. The objective of geometry optimization is to achieve a structure in which all the atoms are at equilibrium, that is, one in which the forces acting on every atom are very small, and in which the second derivatives are everywhere positive. Such a geometry is called a ground state stationary point.