Without loss of generality, take x to be a fixed-size (among all NPMs) vector representation of the NPM that lies in a continuous feature space. The NPM feature vectors {x} should be designed to ( ) encode the relevant structural and chemical features of the NPMs, ( ) be rotation-, translation-, and, if the NPM is a crystal, replication-invariant, ( ) be cheap to compute compared to conducting the experiment (evaluating f ), and ( ) ideally, each correspond to a unique NPM (mapping NPM → x is injective) [ ]. As a result, NPMs close in the feature space should exhibit close values of the property y .