- The effects of interstitial carbon on the diffusion and mechanical properties of copper and silver are studied theoretically. Semiempirical methodology, atomistic simulations, and first-principles density functional schemes are combined to extract some understanding of the diffusion process and lattice reconstruction in extremely dilute interstitial Cu–C and Ag–C alloys. It is demonstrated that carbon inclusion in the host matrix leads to sufficient non-uniform dilatation of the lattice. We also show that an account of static displacements is important in the calculations of the activation energy for the diffusion of the interstitial atoms. The “embedded” cluster scheme is suggested to simulate the relaxation in extremely dilute alloys. High-resolution scanning electron microscopy results are presented, which demonstrate the existence of a solid solution zone at the Cu–C interface.