Finding transition states using contangency curves

We present a new method for approximating reaction paths and transition states of conformational transitions in polyatomic systems. The method uses the quasi-harmonic properties of the metastable state to construct a bi-Gaussian model for the energy landscape. Its reaction path is approximated by the equipotential contour cotangency (contangency) curve which connects the two equilibrium states with the saddle point lying between them. Unlike the reaction path, the contangency curve has an explicit analytic definition which makes it a useful starting point for conventional reaction path calculations. The method is illustrated for the conformational transition between the chair and twisted-boat isomers of cyclohexane.