Theophilou, I., Lathiotakis, N. N., Gidopoulos, N. I., Rubio, A., Helbig, N.

Orbitals from local RDMFT: Are they Kohn-Sham or natural orbitals?
Journal of Chemical Physics **143**, (5),pp 054106/1-7 (2015)
Recently, an approximate theoretical framework was introduced, called local reduced density matrix
functional theory (local-RDMFT), where functionals of the one-body reduced density matrix
(1-RDM) are minimized under the additional condition that the optimal orbitals satisfy a single
electron Schrödinger equation with a local potential. In the present work, we focus on the character
of these optimal orbitals. In particular, we compare orbitals obtained by local-RDMFT with those
obtained with the full minimization (without the extra condition) by contrasting them against the
exact NOs and orbitals from a density functional calculation using the local density approximation
(LDA). We find that the orbitals from local-RMDFT are very close to LDA orbitals, contrary to
those of the full minimization that resemble the exact NOs. Since local RDMFT preserves the
good quality of the description of strong static correlation, this finding opens the way to a mixed
density/density matrix scheme, where Kohn-Sham orbitals obtain fractional occupations from a
minimization of the occupation numbers using 1-RDM functionals. This will allow for a description
of strong correlation at a cost only minimally higher than a density functional calculation.

TH-2015-23