In the typical use-cases of GB-BI, the descriptor functions are physicochemical properties which can be efficiently calculated, so that we can make use of a simplified version of BOP-Elites in which niche occupancy is determined exactly and a surrogate model is only constructed for the fitness function. As acquisition function, BOP-Elites uses the expected improvement (EI) which considers both the probability of improving on the current solution in a niche and the magnitude of the predicted improvement. In the case where descriptor functions are calculated exactly, this acquisition function boils down to