The cohesive energy (CE) of CoPt nanoparticles (NPs) with different sizes and shapes have been calculated by embedded-atom-method (EAM) potential. It is shown that CE of NPs with order or disorder structures decreases with the decrease of particle size, while the shape effects become obvious only at small size. The CE difference per atom between order and disorder structures decreases with the decrease of particle size, indicating that the possibility of order-disorder transition in small size becomes larger compared with these in large size. Significantly, the CE difference varies in proportion to order-disorder transition temperature (T(c)), which suggests that one can predict order-disorder transition of NPs by calculation the cohesive energy. The present calculated T(c) of CoPt NPs is consistent with recent experiments, simulation and theoretical predictions, and the method can also be applied to study the order-disorder transition of FePt, FePd, and so on.
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