## Abstract

In many of the approximate functionals in one-body reduced-density-matrix (1RDM) functional theory, the approximate two-body reduced density matrix (2RDM) in the natural orbital representation only depends on the natural occupation numbers. In Phys. Rev. A 92, 012520 (2015)PLRAAN1050-294710.1103/PhysRevA.92.012520, Wang and Knowles initialized a discussion of to what extent this simplification is valid by introducing two different H4 geometries with identical natural occupation numbers but different 2RDMs. Gritsenko has argued that this feature is due symmetry [Phys. Rev. A 97, 026501 (2018)2469-992610.1103/PhysRevA.97.026501]. This work aims to contribute to the discussion on the following points: (1) one should rather speak of symmetry-restricted variants of the universal functional than saying that the universal functional is symmetry dependent; (2) the unitary invariance of degenerate NOs can lead to large deviations in the 2RDM elements, especially the phase of the NOs; (3) symmetry-restricted functionals are constructed for the H4 geometries considered by Wang and Knowles, whose structure could serve as guide in the construction of approximate 1RDM functionals.

Original language | English |
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Article number | 052814 |

Pages (from-to) | 1-15 |

Number of pages | 15 |

Journal | Physical Review A |

Volume | 102 |

Issue number | 5 |

DOIs | |

Publication status | Published - 16 Nov 2020 |

### Funding

The author would like to thank Prof. Dr. E. J. Baerends and Dr. O. V. Gritsenko for useful comments and discussions. The author acknowledges funding by Stichting voor Fundamenteel Onderzoek der Materie FOM Projectruimte (Project No. 15PR3232) and European Research Council under H2020/ERC Consolidator Grant "corr-DFT" (Grant No. 648932).

Funders | Funder number |
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Horizon 2020 Framework Programme | 648932 |

European Research Council | |

Stichting voor Fundamenteel Onderzoek der Materie | 15PR3232 |