Zero-curvature tunneling

Alternative corrections are Eckart tunneling, multidimensional zero-curvature tunneling, and multidimensional small-curvature tunneling (Truhlar et al. 1982), in increasing order of accuracy. The last two involve points on the PES other than the first order saddle point and have more sophisticated calculations. 

The QFH potential approximately captures two key quantum effects. When an atom is near a potential minimum, the curvature is positive and thus so is the QFH correction this models the zero-point effect. On the other hand, near potential maxima the curvature is negative, and the QFH potential models tunneling. 

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