Add new preprint reference to papers.markdown

Author: aronwalsh

papers.markdown

@@ -13,6 +13,7 @@ permalink: /papers/
 
 # Preprints
 * "Guidelines for robust and reproducible point defect simulations in crystals" [ChemRxiv (2025)](https://chemrxiv.org/engage/chemrxiv/article-details/68d28f6ef2aff16770238309)
+* "MLIP Arena: Advancing fairness and transparency in machine learning interatomic potentials via an open, accessible benchmark platform" [arXiv (2025)](https://arxiv.org/abs/2509.20630)
 * "Dynamic vacancy levels in CsPbCl<sub>3</sub> obey equilibrium defect thermodynamics" [arXiv (2025)](https://arxiv.org/abs/2508.14513)
 * "Closing the synthesis gap in computational materials design" [ChemRxiv (2025)](https://chemrxiv.org/engage/chemrxiv/article-details/687ce72323be8e43d628846f)
 * "Phase stability and transformations in lead mixed halide perovskites from machine learning force fields" [arXiv (2025)](https://arxiv.org/abs/2507.07926)