Discontinuity of the exchange-correlation potential and the functional derivative of the noninteracting kinetic energy as the number of electrons crosses integer boundaries in Li, Be, and B.

Accurate densities were determined from configuration interaction wave functions for atoms and ions of Li, Be, and B with up to four electrons. Exchange-correlation potentials, Vxc(r), and functional derivatives of the noninteracting kinetic energy, δK[p]/δp(r), obtained from these densities were used to examine their discontinuities as the number of electrons N increases across integer boundaries for N = 1, N = 2, and N = 3. These numerical results are consistent with conclusions that the discontinuities are characterized by a jump in the chemical potential while the shape of V_xc(r) varies continuously as an integer boundary is crossed. The discontinuity of the V_xc(r) is positive, depends on the ionization potential, electron affinity, and orbital energy differences, and the discontinuity in δK[p]/δp(r) depends on the difference between the energies of the highest occupied and lowest unoccupied orbitals. The noninteracting kinetic energy and the exchange correlation energy have been computed for integer and noninteger values of N between 1 and 4. [ABSTRACT FROM AUTHOR]