The scanning tunnelling microscope (STM) has been used to modify surfaces with atomic resolution and has consequently been proposed as a tool for nanotechnology. Here we examine a process for deposition of (100) diamond under a localized electric field. We use ab initio quantum-chemistry techniques to investigate the effect of an electric field on the chemical vapour deposition of (100) diamond. The field approximates the field of an STM by using a point charge placed 15 Angstrom above the surface to create a 0.64 V Angstrom (-1) field at the dimer. Our study explores the effect of this electric field on CH(3) adsorption, and the dimer-opening and ring-closing steps of the Brenner-Garrison diamond CVD mechanism. CH(3) adsorption is exothermic by 84 kcal mol(-1) and is not affected by the electric field. The dimer-opening and ring-closing steps are sensitive to the applied field: the dimer-opening barrier is reduced from 9.6 to 6.0 kcal mol(-1), while the barrier of the ring-closing step is reduced from 13.6 to 11.0 kcal mol(-1). Our results indicate that the rate of CVD diamond growth can be enhanced by the application of an electric field, in agreement with experiment.