Quadratic Synchronous Transit QST method

Fig. 4. The linear synchronous transit (LST) and quadratic synchronous transit (QST) methods for finding transition structures R, reactants P, prc ucts TS, true transition structure 1, maximum on LST path (full curveV, 2, minimum perpendicular to LST path 3, maximum on QST path (broken curve). The model surface is constructed from two...

The linear synchronous transit (LST) method searches for an energy maximum along a linear path between reactants and products. The quadratic synchronous transit (QST) method searches for a maximum along a parabola connecting reactants and products, and for a minimum in all directions perpendicular to the parabola. Thus, when properly configured, it is able to locate saddle points on potential energy surfaces. 

The LST and QST calculations do not actually locate a proper transition state but aim to arrive at structures sufficiently close to it. Ideally, the resulting configuration would lie within the quadratic basin of the first order saddle point and be suitable for input to subsequent transition state searches. However, the synchronous transit methods often yield structures with more than one negative eigenvalue. 

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