Large language models (LLMs) are changing the way researchers interact with code and data in scientific computing. While their ability to generate general-purpose code is well established, their effectiveness in producing scientifically valid code/input scripting for domain-specific languages (DSLs) remains largely unexplored. We propose an evaluation procedure that enables domain experts (who may not be experts in the DSL) to assess the validity of LLM-generated input files for LAMMPS, a widely used molecular dynamics (MD) code, and to use those assessments to evaluate the performance of state-of-the-art LLMs and identify common issues. Key to the evaluation procedure are a normalization step to generate canonical files and an extensible parser for syntax analysis. The following steps isolate common errors without incurring costly tests (in time and computational resources). Once a working input file is generated, LLMs can accelerate verification tests. Our findings highlight limitations of LLMs in generating scientific DSLs and a practical path forward for their integration into domain-specific computational ecosystems by domain experts.