Empirically, GPs tend to be effective surrogate models for Bayesian optimization of molecules in the small-data regime. In Methods, we explain the meaning behind the notation of the multi-fidelity GP in eqn. 5, following the Bayesian paradigm 96 of (i) specifying a prior distribution, (ii) collecting the simulation data, then (iii) updating the prior to a posterior distribution. The resulting posterior distribution is Gaussian