A substantial difference between density- and density-matrix- functional theories is that, within the latter, the physical (interacting) problem cannot be mapped onto a pure-state non-interacting one. The reason is simple. A non-interacting one-electron reduced density matrix (1RDM) is idempotent, while the physical one is not. A solution to this issue might be to consider a partially-interacting system instead. In this context, systems with no seniority are good candidates. Indeed, in the Slater determinant expansion of a seniority-zero wavefunction, the orbitals are, by construction, the natural ones. The physical 1RDM can then be recovered exactly if the configuration interaction (CI) coefficients are adjusted such that the true natural orbital occupancies are reproduced. An in-principle-exact variational formulation of the theory will be presented. The resulting (coupled) self-consistent equations for both natural orbitals and CI coefficients will also be discussed. In order to get further insight into the 1RDM functional that complements the seniority-zero energy, an adiabatic connection (AC) formalism will then be introduced. Accurate AC curves obtained for prototypical systems (with the density matrix renormalization group method) will be presented and analyzed.

Mapping the interacting electronic problem onto a seniority-zero one in density-matrix functional theory

2019 Spring National Meeting

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