628 research outputs found
Quasi-pinning and entanglement in the lithium isoelectronic series
The Pauli exclusion principle gives an upper bound of 1 on the natural
occupation numbers. Recently there has been an intriguing amount of theoretical
evidence that there is a plethora of additional generalized Pauli restrictions
or (in)equalities, of kinematic nature, satisfied by these numbers. Here for
the first time a numerical analysis of the nature of such constraints is
effected in real atoms. The inequalities are nearly saturated, or quasi-pinned.
For rank-six and rank-seven approximations for lithium, the deviation from
saturation is smaller than the lowest occupancy number. For a rank-eight
approximation we find well-defined families of saturation conditions.Comment: 22 pages, 6 figures, minor changes, references adde
Relating correlation measures: the importance of the energy gap
The concept of correlation is central to all approaches that attempt the
description of many-body effects in electronic systems. Multipartite
correlation is a quantum information theoretical property that is attributed to
quantum states independent of the underlying physics. In quantum chemistry,
however, the correlation energy (the energy not seized by the Hartree-Fock
ansatz) plays a more prominent role. We show that these two different
viewpoints on electron correlation are closely related. The key ingredient
turns out to be the energy gap within the symmetry-adapted subspace. We then
use a few-site Hubbard model and the stretched H to illustrate this
connection and to show how the corresponding measures of correlation compare.Comment: 6 pages, 3 figure
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