Extrema of the density functional for the energy: Excited states from the ground-state theory

Although every stationary-state density n_i(r→) of a many-particle system is not an extremum of the ground-state density functional E_v[n], every extremum of E_v[n] [i.e., every solution of the Euler equation δE_v/δn(r→)=λ] is a stationary-state density n_i(r→). Always, E_v[n_i]≤E_i, where E_i is the lowest stationary-state energy for density n_i(r→); the equality holds if and only if n_i(r→) is an extremum of E_v[n]. The extrema lying above the absolute minimum are excited-state densities which fail to be pure-state v-representable. Surprisingly, infinitesimal number-conserving density variations δn(r→) about an extremum n(r→) do not lead to energy variations δE_v of order (δn)² when δn(r→)/n^1/2(r→) fails to be square-integrable; in fact, variations δE_v of order ‖δn‖ about the ground state are exemplified by the recently discovered ‘‘derivative discontinuities of the energy.’’ This unconventional behavior of E_v[n] may be traced in part to an asymptotic divergence of δ²E_v/δn(r→)δn(r→’). Conditions are presented under which a self-consistent solution of the Kohn-Sham single-particle problem represents an extremum of E_v[n]. The multiplets of the ground-state orbital configuration of the carbon atom are examined. The local-density and Langreth-Mehl approximations are found to yield a remarkably accurate account of the degeneracy of the various ground-state densities for this system, but no estimate of the multiplet splitting is obtained. Finally, aspects of v-representability are discussed, with emphasis on the iron atom.