Unconventional energetics of small vacancy clusters in BCC high-entropy alloy Nb0.75ZrTiV0.5

•Unlike conventional BCC metals, small vacancy clusters do not prefer compact cluster configurations in the studied refractory high-entropy alloy.•The energetics of small vacancy clusters exhibits large site-to-site variation due to severe local lattice distortion and chemical complexity.•The degree of short-range order in the refractory high-entropy alloy has a significant impact on vacancy properties.•Small vacancy clusters in Nb0.75ZrTiV0.5 have a higher probability of dissociation compared to pure metals, which impacts void nucleation, vacancy diffusion, as well as long-term evolution of irradiation defects.