Elastic stretching penalty

One problem with the elastic band method is illustrated in Fig. 6.7. If the penalty in the objective function for stretching one or more of the springs is too low, then images tend to slide downhill, away from the transition state, which of course is precisely the location we want to the highest image to approximate. This difficulty can be reduced, at least in principle, by finding an appropriate stiffness for the spring constants. 

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