The optimization of χ(x) was a global minimization problem; thus, local minima in the parameter space needed to be avoided during the iteration process. We adapted a previously reported stochastic differential evolution (DE) method; 60 this makes it possible to search for the global minimum within a given parameter range at the expense of higher computational cost. The minimum and maximum boundaries for the search space in the DE method were set to be reasonable within the referenced COMPASS database. 33