  We present a high voltage extension of the Tersoff-Hamann theory of STM
images, which includes the effect of the electric field between the tip and the
sample. The theoretical model is based on first principles electronic structure
calculations and has no adjustable parameters. We use the method to calculate
theoretical STM images of the monohydrate Si(100)-H(2$\times$1) surface with
missing hydrogen defects at $- 2$~V and find an enhanced corrugation due to the
electric field, in good agreement with experimental images.
