Collective natural orbital occupancies of harmonium

In the harmonium atom, the collective occupancies {n(l)} of natural orbitals with different angular momenta l can be rigorously studied for those values of the confinement strength omega that lead to ground-state wave functions with correlation factors given by polynomials of kth order. Analysis of two such cases, with omega equal to 1/2 (k=1) and 1/10 (k=2), uncovers some interesting properties of {n(l)}, which turn out to be given by rational functions of pi(1/2) with simple coefficients. In particular, contrary to naive expectations, the collective occupancies for l>2 actually decrease upon strengthening of correlation. The explicit formulas for l-matrices and {n(l)} can be readily utilized in testing, calibration, and benchmarking of approximate implementations of the density matrix functional theory (DMFT). (C) 2005 American Institute of Physics.