Fermionic natural occupation numbers do not only obey Pauli's exclusion principle but are even stronger restricted by so-called generalized Pauli constraints. Whenever given natural occupation numbers lie on the boundary of the allowed region the corresponding N-fermion quantum state has a significantly simpler structure. We recall the recently proposed natural extension of the Hartree–Fock ansatz based on this structural simplification. This variational ansatz is tested for the lithium atom. Intriguingly, the underlying mathematical structure yields universal geometrical bounds on the correlation energy reconstructed by this ansatz.
We are grateful to José M. Gracia-Bondía, Dieter Jaksch, Nektarios Lathiotakis, Miguel Marques and Vlatko Vedral for helpful discussions. Financial supported is acknowledged from the Colombian Department of Sciences and Technology (CBR) and from the Swiss National Science Foundation (Grant P2EZP2 152190) and the Oxford Martin Programme on Bio-Inspired Quantum Technologies (CS).
©2016 Walter de Gruyter Berlin/Boston