cs.CLApr 8, 2026

ChemVLR: Prioritizing Reasoning in Perception for Chemical Vision-Language Understanding

Authors: Xuanle Zhao, Xinyuan Cai, Xiang Cheng, Xiuyi Chen, Bo Xu

Organizations: The Key Laboratory of Cognition and Decision Intelligence for Complex Systems, Institute of Automation, Chinese Academy of Sciences · School of Artificial Intelligence, University of Chinese Academy of Sciences

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Abstract

While Vision-Language Models (VLMs) have demonstrated significant potential in chemical visual understanding, current models are predominantly optimized for direct visual question-answering tasks. This paradigm often results in "black-box" systems that fail to utilize the inherent capability of Large Language Models (LLMs) to infer underlying reaction mechanisms. In this work, we introduce ChemVLR, a chemical VLM designed to prioritize reasoning within the perception process. Unlike conventional chemical VLMs, ChemVLR analyzes visual inputs in a fine-grained manner by explicitly identifying granular chemical descriptors, such as functional groups, prior to generating answers. This approach ensures the production of explicit and interpretable reasoning paths for complex visual chemical problems. To facilitate this methodology, we implement a cross-modality reverse-engineering strategy, combined with a rigorous filtering pipeline, to curate a large-scale reasoning-and-captioning dataset comprising 760k high-quality samples across molecular and reaction tasks. Furthermore, we adopt a three-stage training framework that systemically builds model perception and reasoning capacity. Experiments demonstrate that ChemVLR achieves state-of-the-art (SOTA) performance, surpassing both leading proprietary models and domain-specific open-source baselines. We also provide comprehensive ablation studies to validate our training strategy and data generation designs. Code and model weights will be available at https://github.com/xxlllz/ChemVLR.

Figures & tables

Figure 1: Overview of our proposed Cross-Modality Reverse-Engineering strategy for data generation. The pipeline begins with an initial dataset constructed from textual SMILES QA pairs, enriched with auxiliary information, including IUPAC names, functional groups, and expert demonstrations. Once Gemini-2.5-Flash generates the reasoning and caption data, they are subjected to a rigorous three-stage filtering and verification process to ensure accuracy and quality. Subsequently, the filtered data are rendered into images using RDKit and Indigo code.
SettingMol. Rec.Rxn. Rec.Rxn. Pred.
SMILES78.0%58.0%55.0%
+ Demo80.0%64.0%62.0%
+ IUPAC92.0%71.0%71.0%
+ RDKit95.0%73.0%76.8%
Table 1: Effectiveness comparison of various data generation settings. We generate 10k samples for each category utilizing our reverse-engineering strategy, except for the last row which utilizes the whole reasoning set. The percentages represent the retention rate of high-quality data following the three-stage filtering protocol.
Figure 2: The training framework of ChemVLR proceeds in three stages. First, we conduct chemical-domain continual pre-training using caption data. Second, during the SFT stage, we train on a mixture of reasoning and instruction data. For the RL stage, we retain only non-trivial yet solvable instances and optimize utilizing DAPO.
Caption DatasetSamplesAverageSD
ChEBI-MM26k151.5780.46
ChemMLLM70k95.9252.46
Molecule (Ours)200k409.93139.03
Reaction (Ours)200k441.0676.85
Table 2: Token Length Statistics of Caption Datasets. The mean and standard deviation (SD) of token counts are computed via the Qwen3-VL tokenizer.
ModelsParadigmMMChemOCRimg2smilesChemRxn-V R{}_{\text{R}}ChemRxn-V P{}_{\text{P}}
Avg Sim.Tani @ 1.0Avg Sim.Tani @ 1.0Tani @ 1.0Avg Sim.Tani @ 1.0
Proprietary Models
zz Gemini-3-Flash-77.661.274.563.84.476.851.7
GPT-5-mini-28.85.720.45.10.724.32.3
GPT-4o-36.83.429.00.10.130.41.4
Open-Source General-Domain Models
Table 3: Combined evaluation on molecular and reaction tasks. Avg Sim. and [email protected] denote Average Tanimoto Similarity and Tanimoto [email protected]. ChemRxn-V R{}_{\text{R}} and ChemRxn-V P{}_{\text{P}} correspond to the reaction recognition and prediction subsets. Bold indicates the best performance.
Figure 3: The reward curve during the RL training process. The overall rewards are the weighted average of format and accuracy rewards.
Figure 4: Showcase of the reasoning process and final answer on the molcular recognition and reaction prediction tasks. The ChemVLR-8B generate the correct answer compare with other baselines.
Ablationimg2smilesRxn-V R{}_{\textbf{R}}Rxn-V P{}_{\textbf{P}}
SFT only81.560.256.8
RL only2.10.20.1
CPT + SFT84.763.559.3
ChemVLR-8B92.774.467.8
Table 4: Ablation study of different training stages. Experiments are conducted using Qwen3-VL-8B-Instruct, with performance evaluated via [email protected]. For brevity, we refer to ChemRxn-V as Rxn-V.
Ablationimg2smilesRxn-V R{}_{\textbf{R}}Rxn-V P{}_{\textbf{P}}
Full SFT Data84.763.562.6
w/o IUPAC80.856.256.1
w/o Reasoning76.453.853.3
w/o Instuction68.446.546.9
Table 5: Ablation study on SFT data composition. The instruction data contains the molecular-to-IUPAC data.
Ablationimg2smilesRxn-V R{}_{\textbf{R}}Rxn-V P{}_{\textbf{P}}
Tani Simarity90.270.865.9
Exact Matching90.869.966.4
Struct-ID (Ours)92.774.467.8
Table 6: Ablation study on different reward functions for RL. We compare our utilized structural identity reward against dense Tanimoto similarity and sparse exact string matching rewards.
Appendix figures & tables9 assets

Supplementary material from the paper’s appendix.

Appendix

Reasoning DataSamplesAverageSD
Mol. Recognition95k811.10307.44
Rxn. Recognition73k937.00233.54
Rxn. Prediction192k1026.40320.98
Appendix
Table 7: Statistics of reasoning thinking lengths in our constructed reasoning datasets. Token counts (mean and standard deviation) are calculated using the Qwen3-VL tokenizer.
Figure 5: Full response of InternVL3.5-38B on the Figure 4 molecular recognition tasks.
Figure 6: Full response of ChemVLR-8B-SFT on the Figure 4 molecular recognition tasks.
Figure 7: Full response of ChemVLR-8B on the Figure 4 molecular recognition tasks.
Figure 8: Full response of InternVL3.5-38B on the Figure 4 reaction prediction tasks.
Figure 9: Full response of ChemVLR-8B on the Figure 4 molecular recognition tasks.
Figure 10: Full response of ChemVLR-8B-SFT on the Figure 4 reaction prediction tasks.
Figure 11: Full response of ChemVLR-8B on the Figure 4 reaction prediction tasks.
Figure 12: The prompt to generate the reasoning process of reaction prediction.
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