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Testing Predictions from Density Functional Theory at Finite Temperatures: $β_2$-Like Ground States in Co-Pt

We perform a critical assessment of the accuracy of DFT-based methods in predicting stable phases within the Co-Pt binary alloy. Statistical mechanical analysis applied to zero kelvin DFT predictions yields finite-temperature results that can be directly compared with experimental measurements. The predicted temperature-composition phase diagram is qualitatively incompatible with experimental observations, indicating that the predicted stability of long-period superstructures as ground states in the Co-Pt binary is incorrect. We also show that recently suggested methods to better align DFT and experiment via the hybrid functional HSE06 are unable to resolve the discrepancies in this system. Our results indicate a need for better verification of DFT based phase stability predictions, and highlight fundamental flaws in the ability of DFT to treat late 3$d$-5$d$ binary alloys.