of vector f θ (t i,1:ℓ-1 , s i ) corresponding to the token t iℓ . The NLL represents the probability (precedence) of transforming a given source molecule into a specific target molecule. The NLL is always non-negative and the higher the NLL value, the less likely a target molecule will be generated. A NLL equal to 0.0 would imply that a specific target molecule would have the probability of 1.0 to be generated from the source molecule.