  This Communication reports a geometrical factor that is necessary in the
diffusion boundary condition across surface steps. Specifically, this factor
relates adatom concentration to its spatial gradient at a surface step, and it
describes the fraction of jump attempts that cross the step. In this
Communication, the authors show that the factor is 1/\Pi using theoretical
formulation and further verify the formulation using numerical simulations for
triangular, square, and hexagonal surface lattices.
