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The interdiffusion processes in liquid Al–Cu and Ni–Cu alloys are studied by using molecular dynamics simulation method. The Maxwell–Stefan (MS) diffusivities are calculated over a wide composition range with both the Green–Kubo method and the Darken relation. Comparisons show that the Darken relation predicts well the MS diffusivity for Ni–Cu alloy, while overestimates the value for Al–Cu alloy, especially in the medium concentration region. Based on the calculated MS diffusivities and the activities of the alloys, the Fickian interdiffusivities are predicted. The results show strong dependences on the compositions of the alloys. In addition, the Fickian interdiffusivities of Al60Cu40 and Ni50Cu50 melts as a function of undercooling are estimated, which is proved to be beneficial in improving the quantitative predictions of dendrite growth velocity in solidification. [ABSTRACT FROM AUTHOR]