The model building procedures is as follow: first the initial set of structures is split into two sub-sets with 80% of the structures utilized as model building set and the remainder 20% utilized as a holdout test set. The holdout test set is kept aside and is not used in the development of QSAR models. Instead, it is utilized as control set to assess the accuracy of the models as the chemicals in the holdout set are external with respect to the model building procedure. The model building set is subject to a fivefold cross-validation procedure for internal validations. In each of the five iterations of this approach, 80% of the model building set is used to build a model, and 20% is held as a test set. Across the entire process, then, every record is held out as validation in one part of the process, yet all records are made available to the model. Grid search was used as the default method for hyperparameter optimization.