The acquisition function. The acquisition function A(x; f (x)) : X → R scores the utility of, next, evaluating NPM x ∈ X with the expensive objective function f . Here, "utility" is defined in terms of our ultimate goal of finding the optimal NPM x * in eqn. with the fewest experiments. The acquisition function employs the prediction of the property ŷ and the associated uncertainty σ 2 from the surrogate model f (x) to assign a utility score to the NPM that balances exploitation and exploration, respectively. Maxima of the acquisition function are located in regions of NPM feature space where the predicted property is large and/or uncertainty is high.