title: Bounds on the overlap of the Hartree-Fock, optimized effective potential, and density functional approximations with the exact energy eigenstates.
abstract: In this paper, we examine the limits of accuracy of the single determinant approximations (Hartree-Fock, optimized effective potential, and density functional theory) to the exact energy eigenstates of many electron systems. We show that an approximate Slater determinant of S(z)=M gives maximum accuracy for states with S=M, provided that perturbation theory for the spin up minus spin down potential is applicable. The overlap with the exact energy eigenstates with S not equal M is much smaller. Therefore, for the case that the emphasis is on wave functions, one must use symmetry preserving theories, although this is at the expense of accuracy in energy.
authors.affiliation: Institute of Materials Science, DEMOCRITOS NCSR, TT 15310 Athens, Greece. sthanos@ims.demokritos.gr
authors.forename: S
authors.initials: S
authors.lastname: Thanos
authors.forename: A K
authors.initials: AK
authors.lastname: Theophilou