  Automatic design of organic materials requires black-box optimization in a
vast chemical space. In conventional molecular design algorithms, a molecule is
built as a combination of predetermined fragments. Recently, deep neural
network models such as variational auto encoders (VAEs) and recurrent neural
networks (RNNs) are shown to be effective in de novo design of molecules
without any predetermined fragments. This paper presents a novel python library
ChemTS that explores the chemical space by combining Monte Carlo tree search
(MCTS) and an RNN. In a benchmarking problem of optimizing the octanol-water
partition coefficient and synthesizability, our algorithm showed superior
efficiency in finding high-scoring molecules. ChemTS is available at
https://github.com/tsudalab/ChemTS.
