In cases where an optimization over a surrogate model failed to find a lower objective value than the current simulation objective, surrogates are rebuild with constraints on the length scales 𝑙 used for the variances of each parameter. This approach was chosen as we found in testing that optimizations may fail because a surrogate was set with a length scale too low during ARD, producing a surrogate with poor quality. If the optimizer cannot find a better objective value over the surrogate, it is first rebuilt with a length scale constraint such that 𝑙 > 10 -10 ; if this is not successful, a stricter length scale constraint of 𝑙 > 10 -5 is imposed. If this is still not successful, the optimization is terminated. The quality of the surrogate model is reduced when length scale constraints are introduced, so constraints are not used unless an optimization fails.