We employ molecular statics and molecular dynamics computer simulation methods to study structural relaxation of small (⟨10) vacancy and interstitial clusters in Cu. Vacancy clusters whose sizes are below four do not relax much, but clusters larger than five were found to relax appreciably, often into a stacking fault tetrahedron and an octahedral void. It was found that stacking fault tetrahedra of hexa-vacancies relax to a void. Interstitials were introduced into bcc positions. A di-interstitial was found to relax to two parallel ⟨110⟩ split interstitials and tri- and tetra-interstitials relaxed to composite clusters of ⟨100⟩ split interstitials and a bcc interstitial. Penta- and hexa-interstitials form agglomerates of parallel ⟨110⟩ crowdions whose central portions are not on a single (111) plane. Such structures allow easy motion of these clusters along ⟨110⟩ directions under stress. Such movement of interstitial clusters has been observed by electron microscopy in neutron-irradiated Au and Cu.