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Simple hydrogenic estimates for the exchange and correlation energies of atoms and atomic ions, with implications for density functional theory
Kaplan, AD Santra, B Bhattarai, P Wagle, K Chowdhury, STUR Bhetwal, P Yu, J Tang, H Burke, K Levy, M
Kaplan, AD
Santra, B
Bhattarai, P
Wagle, K
Chowdhury, STUR
Bhetwal, P
Yu, J
Tang, H
Burke, K
Levy, M
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2020-08-21
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10.1063/5.0017805
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© 2020 Author(s). Exact density functionals for the exchange and correlation energies are approximated in practical calculations for the ground-state electronic structure of a many-electron system. An important exact constraint for the construction of approximations is to recover the correct non-relativistic large-Z expansions for the corresponding energies of neutral atoms with atomic number Z and electron number N = Z, which are correct to the leading order (-0.221Z5/3 and -0.021Z ln Z, respectively) even in the lowest-rung or local density approximation. We find that hydrogenic densities lead to Ex(N, Z) ≈ -0.354N2/3Z (as known before only for Z ≫ N ≫ 1) and Ec ≈ -0.02N ln N. These asymptotic estimates are most correct for atomic ions with large N and Z ≫ N, but we find that they are qualitatively and semi-quantitatively correct even for small N and N ≈ Z. The large-N asymptotic behavior of the energy is pre-figured in small-N atoms and atomic ions, supporting the argument that widely predictive approximate density functionals should be designed to recover the correct asymptotics. It is shown that the exact Kohn-Sham correlation energy, when calculated from the pure ground-state wavefunction, should have no contribution proportional to Z in the Z → ∞ limit for any fixed N.
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Journal of Chemical Physics
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