A surface, usually taken to be a hyperplane, constructed at right angles to the @M03931@ on a @P04780@. In conventional @T06470@ it passes through the highest point on the minimum-energy path. In generalized versions of transition-state theory the dividing surface can be at other positions; in @V06603@ the position of the dividing surface is varied so as to get a better estimate of the @O04322@.