Similarity to a nearest neighbor (SNN) : computes average Tanimoto similarity between fingerprints of a molecule from the generated set G and its nearest neighbor molecule in the reference set R. The metric is limited by [0, 1]. If generated molecules are far from the manifold of the reference set, the similarity to the nearest neighbor will be low. In MOSES, Tanimoto similarity is based on standard Morgan ECFP with radius 2 and 1024 bits computed using RDKit.