Even though the intrinsic reaction coordinate (IRC) is defined in mass-weighted cartesian coordinates, other choices exist to follow the minimum energy reaction pathway in Gaussian. The default selection of mass-weighted cartesian coordinates need not, but can be specified more explicitly with

irc=MassWeighted or irc=MW

Other options include the calculation in cartesian coordinates without mass-weighing, giving significantly different results in systems in which light and heavy elements are directly connected (e.g. hydrogen atom transfer reactions). This choice is selected with


A third choice uses an internal coordinate system as given in the input file:


In some rare cases, reaction path calculations lead to one product in mass-weighted cartesian coordinates and to another product in cartesian coordinates (without mass-weighing). In most cases, however, the same products are reached in either coordinate system.