Use and abuse of the distinguished-coordinate method for transition-state structure searching

Exploration of potential-energy surfaces for reacting systems is often expedited by selection of a distinguished coordinate, to measure progress between reactants and products, at each value of which the remaining coordinates are optimized by constrained energy-minimization. A classification of surfaces is suggested according to the number of energy minima in the subspace excluding the distinguished coordinate in the saddle region. Type S surfaces are well behaved in the distinguished-coordinate method but type D surfaces are not. In general, reaction paths and energy profiles generated by this method for type D surfaces are discontinuous and cannot be used to locate transition-state structures precisely. An extension of the method using two distinguished coordinates is discussed in relation to the example of formaldehyde hydration but is found to suffer from the same pathological condition in two dimensions as does the standard method in one dimension.