Explicit asymptotic expressions are derived for the reduced one-electron density matrices (the 1-matrices) of strongly correlated two-and three-electron harmonium atoms in the ground and first excited states. These expressions, which are valid at the limit of small confinement strength omega, yield electron densities and kinetic energies in agreement with the published values. In addition, they reveal the omega(5/6) asymptotic scaling of the exchange components of the electron-electron repulsion energies that differs from the omega(2/3) scaling of their Coulomb and correlation counterparts. The natural orbitals of the totally symmetric ground state of the two-electron harmonium atom are found to possess collective occupancies that follow a mixed power/Gaussian dependence on the angular momentum in variance with the simple power-law prediction of Hill's asymptotics. Providing rigorous constraints on energies as functionals of 1-matrices, these results are expected to facilitate development of approximate implementations of the density matrix functional theory and ensure their proper description of strongly correlated systems. (C) 2015 AIP Publishing LLC.
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