Evaluating Universal Machine Learning Force Fields Against Experimental Measurements

Universal machine learning force fields (UMLFFs) promise to revolutionize materials science by enabling rapid atomistic simulations across the periodic table. However, their evaluation has been limited to computational benchmarks that may not reflect real-world performance. Here, we present UniFFBench, a comprehensive framework for evaluating UMLFFs against experimental measurements of ~1,500 carefully curated mineral structures spanning diverse chemical environments, bonding types, structural complexity, and elastic properties. Our systematic evaluation of six state-of-the-art UMLFFs reveals a substantial reality gap: models achieving impressive performance on computational benchmarks often fail when confronted with experimental complexity. Even the best-performing models exhibit higher density prediction error than the threshold required for practical applications. Most strikingly, we observe disconnects between simulation stability and mechanical property accuracy, with prediction errors correlating with training data representation rather than the modeling method. These findings demonstrate that while current computational benchmarks provide valuable controlled comparisons, they may overestimate model reliability when extrapolated to experimentally complex chemical spaces. Altogether, UniFFBench establishes essential experimental validation standards and reveals systematic limitations that must be addressed to achieve truly universal force field capabilities.