The gradient metric calculates the score as the absolute value of the difference
in range space (the outputs) of the two points. The output values used are
predicted from the surrogate model.
This method attempts to evenly fill the range space of the surrogate.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.