In order to train a model, you must provide training data containing molecules (as SMILES strings) and known target values. Targets can either be real numbers, if performing regression, or binary (i.e. 0s and 1s), if performing classification. Target values which are unknown can be left as blanks.

Our model can either train on a single target (“single tasking”) or on multiple targets simultaneously (“multi-tasking”).

The data file must be be a CSV file with a header row. For example:


By default, it is assumed that the SMILES are in the first column and the targets are in the remaining columns. However, the specific columns containing the SMILES and targets can be specified using the --smiles_column <column> and --target_columns <column_1> <column_2> ... flags, respectively.

Datasets from MoleculeNet and a 450K subset of ChEMBL from http://www.bioinf.jku.at/research/lsc/index.html have been preprocessed and are available in data.tar.gz. To uncompress them, run tar xvzf data.tar.gz.


To train a model, run:

chemprop_train --data_path <path> --dataset_type <type> --save_dir <dir>

where <path> is the path to a CSV file containing a dataset, <type> is either “classification” or “regression” depending on the type of the dataset, and <dir> is the directory where model checkpoints will be saved.

For example:

chemprop_train --data_path data/tox21.csv --dataset_type classification --save_dir tox21_checkpoints

A full list of available command-line arguments can be found in Command Line Arguments.

If installed from source, chemprop_train can be replaced with python train.py.


  • The default metric for classification is AUC and the default metric for regression is RMSE. Other metrics may be specified with --metric <metric>.

  • --save_dir may be left out if you don’t want to save model checkpoints.

  • --quiet can be added to reduce the amount of debugging information printed to the console. Both a quiet and verbose version of the logs are saved in the save_dir.

Train/Validation/Test Splits

Our code supports several methods of splitting data into train, validation, and test sets.

Random: By default, the data will be split randomly into train, validation, and test sets.

Scaffold: Alternatively, the data can be split by molecular scaffold so that the same scaffold never appears in more than one split. This can be specified by adding --split_type scaffold_balanced.

Separate val/test: If you have separate data files you would like to use as the validation or test set, you can specify them with --separate_val_path <val_path> and/or --separate_test_path <test_path>.

Note: By default, both random and scaffold split the data into 80% train, 10% validation, and 10% test. This can be changed with --split_sizes <train_frac> <val_frac> <test_frac>. For example, the default setting is --split_sizes 0.8 0.1 0.1. Both also involve a random component and can be seeded with --seed <seed>. The default setting is --seed 0.

Cross validation

k-fold cross-validation can be run by specifying --num_folds <k>. The default is --num_folds 1.


To train an ensemble, specify the number of models in the ensemble with --ensemble_size <n>. The default is --ensemble_size 1.

Hyperparameter Optimization

Although the default message passing architecture works quite well on a variety of datasets, optimizing the hyperparameters for a particular dataset often leads to marked improvement in predictive performance. We have automated hyperparameter optimization via Bayesian optimization (using the hyperopt package), which will find the optimal hidden size, depth, dropout, and number of feed-forward layers for our model. Optimization can be run as follows:

chemprop_hyperopt --data_path <data_path> --dataset_type <type> --num_iters <n> --config_save_path <config_path>

where <n> is the number of hyperparameter settings to try and <config_path> is the path to a .json file where the optimal hyperparameters will be saved.

If installed from source, chemprop_hyperopt can be replaced with python hyperparameter_optimization.py.

Once hyperparameter optimization is complete, the optimal hyperparameters can be applied during training by specifying the config path as follows:

chemprop_train --data_path <data_path> --dataset_type <type> --config_path <config_path>

Note that the hyperparameter optimization script sees all the data given to it. The intended use is to run the hyperparameter optimization script on a dataset with the eventual test set held out. If you need to optimize hyperparameters separately for several different cross validation splits, you should e.g. set up a bash script to run hyperparameter_optimization.py separately on each split’s training and validation data with test held out.

Additional Features

While the model works very well on its own, especially after hyperparameter optimization, we have seen that additional features can further improve performance on certain datasets. The additional features can be added at the atom-, bond, or molecule-level. Molecule-level features can be either automatically generated by RDKit or custom features provided by the user.

Molecule-Level RDKit 2D Features

As a starting point, we recommend using pre-normalized RDKit features by using the --features_generator rdkit_2d_normalized --no_features_scaling flags. In general, we recommend NOT using the --no_features_scaling flag (i.e. allow the code to automatically perform feature scaling), but in the case of rdkit_2d_normalized, those features have been pre-normalized and don’t require further scaling.

The full list of available features for --features_generator is as follows.

morgan is binary Morgan fingerprints, radius 2 and 2048 bits. morgan_count is count-based Morgan, radius 2 and 2048 bits. rdkit_2d is an unnormalized version of 200 assorted rdkit descriptors. Full list can be found at the bottom of our paper: https://arxiv.org/pdf/1904.01561.pdf rdkit_2d_normalized is the CDF-normalized version of the 200 rdkit descriptors.

Molecule-Level Custom Features

If you install from source, you can modify the code to load custom features as follows:

  1. Generate features: If you want to generate features in code, you can write a custom features generator function in chemprop/features/features_generators.py. Scroll down to the bottom of that file to see a features generator code template.

  2. Load features: If you have features saved as a numpy .npy file or as a .csv file, you can load the features by using --features_path /path/to/features. Note that the features must be in the same order as the SMILES strings in your data file. Also note that .csv files must have a header row and the features should be comma-separated with one line per molecule.

Atom-Level Features

Similar to the additional molecular features described above, you can also provide additional atomic features via --atom_descriptors_path /path/to/features with valid file formats:

  • .npz file, where descriptors are saved as 2D array for each molecule in the exact same order as the SMILES strings in your data file.

  • .pkl / .pckl / .pickle containing a pandas dataframe with smiles as index and numpy array of descriptors as columns.

  • .sdf containing all mol blocks with descriptors as entries.

The order of the descriptors for each atom per molecule must match the ordering of atoms in the RDKit molecule object. Further information on supplying atomic descriptors can be found here.

Users must select in which way atom descriptors are used. The command line option --atom_descriptors descriptor concatenates the new features to the embedded atomic features after the D-MPNN with an additional linear layer. The option --atom_descriptors feature concatenates the features to each atomic feature vector before the D-MPNN, so that they are used during message-passing. Alternatively, the user can overwrite the default atom features with the custom features using the option --overwrite_default_atom_features.

Similar to the molecule-level features, the atom-level descriptors and features are scaled by default. This can be disabled with the option --no_atom_descriptor_scaling

Bond-Level Features

Bond-level features can be provided in the same format as the atom-level features, using the option --bond_features_path /path/to/features. The order of the features for each molecule must match the bond ordering in the RDKit molecule object.

The bond-level features are concatenated with the bond feature vectors before the D-MPNN, such that they are used during message-passing. Alternatively, the user can overwrite the default bond features with the custom features using the option --overwrite_default_bond_features.

Similar to molecule-, and atom-level features, the bond-level features are scaled by default. This can be disabled with the option --no_bond_features_scaling.


As an alternative to molecule SMILES, Chemprop can also process atom-mapped reaction SMILES (see Daylight manual for details on reaction SMILES), which consist of three parts denoting reactants, agents and products, separated by “>”. Use the option --reaction to enable the input of reactions, which transforms the reactants and products of each reaction to the corresponding condensed graph of reaction and changes the initial atom and bond features to hold information from both the reactant and product (option --reaction_mode reac_prod), or from the reactant and the difference upon reaction (option --reaction_mode reac_diff, default) or from the product and the difference upon reaction (option --reaction_mode prod_diff). In reaction mode, Chemprop thus concatenates information to each atomic and bond feature vector, for example, with option --reaction_mode reac_prod, each atomic feature vector holds information on the state of the atom in the reactant (similar to default Chemprop), and concatenates information on the state of the atom in the product, so that the size of the D-MPNN increases slightly. Agents are discarded. Functions incompatible with a reaction as input (scaffold splitting and feature generation) are carried out on the reactants only. If the atom-mapped reaction SMILES contain mapped hydrogens, enable explicit hydrogens via --explicit_h. Example of an atom-mapped reaction SMILES denoting the reaction of methanol to formaldehyde without hydrogens: [CH3:1][OH:2]>>[CH2:1]=[O:2] and with hydrogens: [C:1]([H:3])([H:4])([H:5])[O:2][H:6]>>[C:1]([H:3])([H:4])=[O:2].[H:5][H:6]. The reactions do not need to be balanced and can thus contain unmapped parts, for example leaving groups, if necessary. For further details and benchmarking, as well as a citable reference, please see DOI 10.33774/chemrxiv-2021-frfhz.


An existing model, for example from training on a larger, lower quality dataset, can be used for parameter-initialization of a new model by providing a checkpoint of the existing model using either:

  • --checkpoint_dir <dir> Directory where the model checkpoint(s) are saved (i.e. --save_dir during training of the old model). This will walk the directory, and load all .pt files it finds.

  • --checkpoint_path <path> Path to a model checkpoint file (.pt file).

when training the new model. The model architecture of the new model should resemble the architecture of the old model - otherwise some or all parameters might not be loaded correctly. Please note that the old model is only used to initialize the parameters of the new model, but all parameters remain trainable (no frozen layers). Depending on the quality of the old model, the new model might only need a few epochs to train.

Missing target values

When training multitask models (models which predict more than one target simultaneously), sometimes not all target values are known for all molecules in the dataset. Chemprop automatically handles missing entries in the dataset by masking out the respective values in the loss function, so that partial data can be utilized, too. The loss function is rescaled according to all non-missing values, and missing values furthermore do not contribute to validation or test errors. Training on partial data is therefore possible and encouraged (versus taking out datapoints with missing target entries). No keyword is needed for this behavior, it is the default.

In contrast, when using sklearn_train.py (a utility script provided within Chemprop that trains standard models such as random forests on Morgan fingerprints via the python package scikit-learn), multi-task models cannot be trained on datasets with partially missing targets. However, one can instead train individual models for each task (via the argument --single_task), where missing values are automatically removed from the dataset. Thus, the training still makes use of all non-missing values, but by training individual models for each task, instead of one model with multiple output values. This restriction only applies to sklearn models (via sklearn_train or python sklearn_train.py), but NOT to default Chemprop models via chemprop_train or python train.py.


By default, the molecule objects created from each SMILES string are cached for all dataset sizes, and the graph objects created from each molecule object are cached for datasets up to 10000 molecules. If memory permits, you may use the keyword --cache_cutoff inf to set this cutoff from 10000 to infinity to always keep the generated graphs in cache (or to another integer value for custom behavior). This may speed up training (depending on the dataset size, molecule size, number of epochs and GPU support), since the graphs do not need to be recreated each epoch, but increases memory usage considerably. Below the cutoff, graphs are created sequentially in the first epoch. Above the cutoff, graphs are created in parallel (on --num_workers <int> workers) for each epoch. If training on a GPU, training without caching and creating graphs on the fly in parallel is often preferable. On CPU, training with caching if often preferable for medium-sized datasets and a very low number of CPUs. If a very large dataset causes memory issues, you might turn off caching even of the molecule objects via the commands --no_cache_mol to reduce memory usage further.


To load a trained model and make predictions, run predict.py and specify:

  • --test_path <path> Path to the data to predict on.

  • A checkpoint by using either:

    • --checkpoint_dir <dir> Directory where the model checkpoint(s) are saved (i.e. --save_dir during training). This will walk the directory, load all .pt files it finds, and treat the models as an ensemble.

    • --checkpoint_path <path> Path to a model checkpoint file (.pt file).

  • --preds_path Path where a CSV file containing the predictions will be saved.

For example:

chemprop_predict --test_path data/tox21.csv --checkpoint_dir tox21_checkpoints --preds_path tox21_preds.csv


chemprop_predict --test_path data/tox21.csv --checkpoint_path tox21_checkpoints/fold_0/model_0/model.pt --preds_path tox21_preds.csv

If installed from source, chemprop_predict can be replaced with python predict.py.


It is often helpful to provide explanation of model prediction (i.e., this molecule is toxic because of this substructure). Given a trained model, you can interpret the model prediction using the following command:

chemprop_interpret --data_path data/tox21.csv --checkpoint_dir tox21_checkpoints/fold_0/ --property_id 1

If installed from source, chemprop_interpret can be replaced with python interpret.py.

The output will be like the following:

  • The first column is a molecule and second column is its predicted property (in this case NR-AR toxicity).

  • The third column is the smallest substructure that made this molecule classified as toxic (which we call rationale).

  • The fourth column is the predicted toxicity of that substructure.

As shown in the first row, when a molecule is predicted to be non-toxic, we will not provide any rationale for its prediction.



















Chemprop’s interpretation script explains model prediction one property at a time. --property_id 1 tells the script to provide explanation for the first property in the dataset (which is NR-AR). In a multi-task training setting, you will need to change --property_id to provide explanation for each property in the dataset.

For computational efficiency, we currently restricted the rationale to have maximum 20 atoms and minimum 8 atoms. You can adjust these constraints through --max_atoms and --min_atoms argument.

Please note that the interpreting framework is currently only available for models trained on properties of single molecules, that is, multi-molecule models generated via the --number_of_molecules command are not supported.


During training, TensorBoard logs are automatically saved to the same directory as the model checkpoints. To view TensorBoard logs, run tensorboard --logdir=<dir> where <dir> is the path to the checkpoint directory. Then navigate to http://localhost:6006.

Web Interface

For those less familiar with the command line, Chemprop also includes a web interface which allows for basic training and predicting. See Web Interface for more details.

Within a python script

Model training and predicting can also be embedded within a python script. To train a model, provide arguments as a list of strings (arguments are identical to command line mode), parse the arguments, and then call chemprop.train.cross_validate():

import chemprop

arguments = [
    '--data_path', 'data/tox21.csv',
    '--dataset_type', 'classification',
    '--save_dir', 'tox21_checkpoints'

args = chemprop.args.TrainArgs().parse_args(arguments)
mean_score, std_score = chemprop.train.cross_validate(args=args, train_func=chemprop.train.run_training)

For predicting with a given model, either a list of smiles or a csv file can be used as input. To use a csv file

import chemprop

arguments = [
    '--test_path', 'data/tox21.csv',
    '--preds_path', 'tox21_preds.csv',
    '--checkpoint_dir', 'tox21_checkpoints'

args = chemprop.args.PredictArgs().parse_args(arguments)
preds = chemprop.train.make_predictions(args=args)

If you only want to use the predictions preds within the script, and not save the file, set preds_path to /dev/null. To predict on a list of smiles, run:

import chemprop

smiles = [['CCC', 'CCCC', 'OCC']]
arguments = [
    '--test_path', '/dev/null',
    '--preds_path', '/dev/null',
    '--checkpoint_dir', 'tox21_checkpoints'

args = chemprop.args.PredictArgs().parse_args(arguments)
preds = chemprop.train.make_predictions(args=args, smiles=smiles)

where the given test_path will be discarded if a list of smiles is provided. If you want to predict multiple sets of molecules consecutively, it is more efficient to only load the chemprop model once, and then predict with the preloaded model (instead of loading the model for every prediction):

import chemprop

arguments = [
    '--test_path', '/dev/null',
    '--preds_path', '/dev/null',
    '--checkpoint_dir', 'tox21_checkpoints'

args = chemprop.args.PredictArgs().parse_args(arguments)

model_objects = chemprop.train.load_model(args=args)

smiles = [['CCC', 'CCCC', 'OCC']]
preds = chemprop.train.make_predictions(args=args, smiles=smiles, model_objects=model_objects)

smiles = [['CCCC', 'CCCCC', 'COCC']]
preds = chemprop.train.make_predictions(args=args, smiles=smiles, model_objects=model_objects)