Hierarchical Graph Tokenization for Molecule-Language Alignment

Recently, there has been a surge of interest in extending the success of large language models (LLMs) from texts to molecules. Most existing approaches adopt a graph neural network to represent a molecule as a series of node tokens for molecule-language alignment, which, however, have overlooked the inherent hierarchical structures in molecules. Notably, higher-order molecular structures contain rich semantics of functional groups, which encode crucial biochemical functionalities of the molecules. We show that neglecting the hierarchical information in tokenization will lead to subpar molecule-language alignment and severe hallucination. To address this limitation, we propose HIerarchical GrapH Tokenization (HIGHT). HIGHT employs a hierarchical graph tokenizer that encodes the hierarchy of atom, motif, and molecular levels of informative tokens to improve the molecular perception of LLMs. HIGHT also adopts an augmented instruction tuning dataset, enriched with the hierarchical graph information, to further enhance the molecule-language alignment. Extensive experiments on 14 real-world benchmarks verify the effectiveness of HIGHT in reducing hallucination by 40%, and significant improvements in various molecule-language downstream tasks. The project is available at https: //higraphllm.github.io/.

How can we teach Large Language Models (LLMs) like ChatGPT to understand graph-structured data like molecules? Molecules are complex, with structures that go beyond individual atoms — they have functional groups and motifs that define their behavior, such as how they interact in biological or chemical processes. Current LLMs that align molecular data with language often overlook these hierarchical structures, leading to hallucinations and misinterpretations, like falsely identifying functional groups.Our paper presents HIGHT, a new technique that captures the hierarchical structure of molecules, from atoms to motifs to the entire molecule. HIGHT uses a hierarchical graph tokenizer to explicitly teach LLMs to recognize and understand the intrinsic hierarchy. Our results show significant improvements in molecule-related tasks, like predicting chemical properties and generating accurate molecule descriptions. Notably, HIGHT reduces errors (or "hallucinations") in identifying functional groups by 40%. This work extends the perception of LLMs to graph-structured data, and lays a foundation for more reliable AI applications in drug discovery, material science, and chemistry.