Model Shepard function

Shepard, J.S. 1993. Using a fractal model to compute the hydraulic conductivity function. Soil Sci. Soc. Am. J. 57 300-306. 

Gradient methods discussed above use a quadratic function (energy, gradient and approximate Hessian) to model the energy surface near the transition state. Distance-weighted interpolants provide a more flexible functional form that can interpolate arbitrarily spaced points with a smooth differentiable function. For a gradient-based optimization, the Shepard interpolation functions seem appropriate... 

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