The quality of diamond thin films on nondiamond substrates is often poor because of large number of defects such as dislocations, twins, stacking faults and grain boundaries. The diamond films often exhibit poor adhesion on nondiamond substrates because of large differences in interatomic potentials between the film and the substrate. The epitaxial nature of the film is determined not only by lattice matching but also by gradient in interatomic potentials across the interface. We have used computer simulations to study energetics of different geometric configurations of C / Si interfaces. The simulations predict that an C / SiC interface had the least energy and therefore the preferred interface configuration. This is in concordance with experimental deposition of C on Si which resulted in formation of an SiC layer in between C and Si.