Probing Chemical Language Models: Effects of Pre-training and Fine-tuning

Anna Karnysheva, Dietrich Klakow, Ji-Ung Lee


Abstract
Chemical language models (CLMs) are trained with linearized representations such as SMILES, yet it remains unclear which chemically meaningful substructures they encode. To foster a better understanding of CLMs, we conduct a systematic study and probe for 78 molecular substructures across eight pre-trained and six randomly initialized models. We furthermore study how fine-tuning on chemical downstream tasks affects the learned representations of molecular substructures. Our results show that pre-training generally improves molecular structure awareness of CLMs, particularly in the upper layers. Moreover, randomly initialized models already encode ring structures well in the first layer. Our analysis on two chemical downstream tasks further reveals that, interestingly, fine-tuning affects task-relevant molecular substructures more than others, indicating that the changes in the representations follow chemical theory.
Anthology ID:
2026.emnlp-main.2662
Volume:
Proceedings of the 2026 Conference on Empirical Methods in Natural Language Processing
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Hung-Yi Lee, Sunipa Dev, Jessy Li, Desmond Elliott
Venue:
EMNLP
SIG:
SIGDAT
Publisher:
Association for Computational Linguistics
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Pages:
60585–60627
Language:
URL:
https://aclanthology.org/2026.emnlp-main.2662/
DOI:
Bibkey:
Cite (ACL):
Anna Karnysheva, Dietrich Klakow, and Ji-Ung Lee. 2026. Probing Chemical Language Models: Effects of Pre-training and Fine-tuning. In Proceedings of the 2026 Conference on Empirical Methods in Natural Language Processing, pages 60585–60627, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
Probing Chemical Language Models: Effects of Pre-training and Fine-tuning (Karnysheva et al., EMNLP 2026)
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https://aclanthology.org/2026.emnlp-main.2662.pdf