The calculation of current is made on the basis of the Extended Huckel theory and the surface Green-function matching method.
The STM image is produced by calculating the currents at different tip positions in the supercell.
We solve the Green's function of the surface and the tip separately, and then match both blocks (only up to first order) at each relative tip-sample position (pixel and tip height). From the Green's function of the combined system we can calculate the elastic current flowing from tip to sample or viceversa. For topographic images, we then readjust at each pixel the tip height until the fixed current value is attained.