We present an investigation of systematic trends for the self-interstitial atom SIA defect behavior in body-centered cubic bcc transition metals using density-functional calculations. In all the nonmagnetic bcc metals the most stable SIA defect configuration has the 111 symmetry. Metals in group 5B of the periodic table V, Nb, Ta have significantly different energies of formation of the 111 and 110 SIA configurations, while for the group 6B metals Cr, Mo, W the two configurations are linked by a soft bending mode. The relative energies of SIA defects in the nonmagnetic bcc metals are fundamentally different from those in ferromagnetic bcc -Fe. The systematic trend exhibited by the SIA defect structures in groups 5B and 6B transition metals correlates with the observed thermally activated mobility of SIA defects.
