Orbitals from local RDMFT: Are they Kohn-Sham or Natural Orbitals?

TL;DR

This paper investigates the nature of orbitals derived from local RDMFT, finding they resemble LDA orbitals more than exact natural orbitals, which suggests new hybrid approaches for describing strong correlation efficiently.

Contribution

It compares orbitals from local-RDMFT with full minimization and exact NOs, revealing their similarity to LDA orbitals and implications for hybrid density/density matrix methods.

Findings

Local-RDMFT orbitals are close to LDA orbitals.

Full minimization orbitals resemble exact natural orbitals.

Potential for hybrid schemes capturing strong correlation efficiently.

Abstract

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 Schr\"odinger 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.