Data set for spin-space averaged Fe–C diffusion calculations with VASP. Output is from two sets of calculations. Monte Carlo simulations of a Heisenberg model yield configurations of individual atomic magnetic moments in bcc Fe at 1043 K with and without an external 6 T field. VASP calculations use these configurations to provide spin-space averaged forces and energies for octahedral and tetrahedral carbon in bcc Fe. Activation energies from these inform diffusion calculations of carbon in bcc Fe at finite temperatures and under field.