  First-principles calculations using density functional theory and two methods
in comparison, Quantum ESPRESSO and Siesta, are done on large supercells which
describe different placements of two adsorbed potassium atoms on the monolayer
of boron carbide BC3. The energy of single K adsorption over the center of C6
ring, of the C4B2 hexagon and over a boron atom are estimated to be 1.66, 1.63
and 1.58 eV, respectively; the comparison of these results with some available
data is discussed. Exact theoretical estimations may be somehow sensitive to
the lattice spacing practically used in the calculation. The interaction of two
potassium atoms adsorbed at close enough distances (less than 10 A) is
negligible if the adsorption occurs at the opposite sides of the BC3 layer, but
creates a steep repulsive potential at distances less than 8 A if the K atoms
are adsorbed on the same side of the monolayer. Relaxation patterns resulting
from the two K atoms being trapped at adjacent adsorption sites in the lattice
are explained. The results suggest that the density of adsorbed K atoms on BC3
can be interestingly high.
