CONJUGATE-GRADIENT CALCULATIONS OF ADATOM INTERACTIONS

We apply an efficient algorithm for density-functional calculations based on conjugate-gradient minimization to the problem of interacting adatoms on a surface. Using a jellium model for sodium, we calculate the interaction energy of two rows of adatoms as a function of separation. We find a long-ranged repulsion which is electrostatic in origin. This repulsive interaction is well modeled by a dipole-dipole repulsion consistent with the large dipole moment of the adatoms.