In contrast, the untrained model hallucinates 100% of the time. As a control, we tested the raw CodeGen-350M-mono model under the same prompts as the fine-tuned version and found that it consistently failed to generate valid crystal structure CIFs, as demonstrated in the SI †. Our tests on CodeGen-350M-mono demonstrate that providing only a prompt to the untrained model results in meaningless outputs, such as True/False or a single number. Including partial output in the prompt was also ineffective. Adding the 'import pymatgen' string caused the model to continue generating import calls to non existing packages. Further guidance using the Structure.from_spacegroup method led to random Pymatgen pseudocalls with incorrect parameter assignments. None of our tests produced a correct structure, which serves to highlight the effect of fine-tuning on performance for our task.