A method for systematically finding the optimal orientation and location of a hyperplanar dividing surface during a transition state theory calculation of a transition rate is presented. The optimization can be carried out during a reversible work evaluation of the free energy barrier. An application to Al adatom diffusion on an Al(100) surface is described. There, the method can converge to give the free energy barrier for the optimal mechanism, a concerted displacement, even when the calculation is initially set up for the less optimal hop mechanism. This illustrates that the method can reveal the optimal mechanism of a transition even when the calculation is started with an incorrect guess.