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The equilibrium vacancy concentration and atomic diffusion coefficients in dilute and complex refractory alloys have been calculated using various computational methods. The most productive technique has been large-scale atomistic simulation in the form of a numerical experiment in which a crystal with free surfaces was simulated for a relatively long time. This method is based on the concept that the free surface acts as a source of point defects and provides a natural way to achieve an equilibrium concentration of the defects within the bulk after an initial annealing stage. For complex concentrated alloys (CCAs), this numerical experiment offers the possibility to study diffusion processes where standard analytical approaches are difficult to apply due to the large variety of microscopic states. As the simulation results, we found that the transition from dilute alloys to CCA is accompanied by a significant increase in atomic diffusion rate due to both a substantial increase in the vacancy concentration and their mobility.