Light-matter hybrid-orbital-based first-principles methods: the influence of the polariton statistics

Journal Of Chemical Theory And Computation 16, 9, 5601 - 5620 (2020)

Light-matter hybrid-orbital-based first-principles methods: the influence of the polariton statistics

Florian Buchholz, Iris Theophilou, Klaas J. H. Giesbertz, Michael Ruggenthaler, Angel Rubio

A detailed understanding of strong matter-photon interactions requires first-principle methods that can solve the fundamental Pauli-Fierz Hamiltonian of non-relativistic quantum electrodynamics efficiently. A possible way to extend well-established electronic-structure methods to this situation is to embed the Pauli-Fierz Hamiltonian in a higher-dimensional light-matter hybrid auxiliary configuration space. In this work we show the importance of the resulting hybrid Fermi-Bose statistics of the polaritons, which are the new fundamental particles of the ``photon-dressed'' Pauli-Fierz Hamiltonian for systems in cavities. We show that violations of these statistics can lead to unphysical results. We present an efficient way to ensure the proper symmetry of the underlying wave functions by enforcing representability conditions on the dressed one-body reduced density matrix. We further present a general prescription how to extend a given first-principles approach to polaritons and as an example introduce polaritonic Hartree-Fock theory. While being a single-reference method in polariton space, polaritonic Hartree-Fock is a multi-reference method in the electronic space, i.e. it describes electronic correlations. We also discuss possible applications to polaritonic QEDFT. We apply this theory to a lattice model and find that the more delocalized the bound-state wave function of the particles is, the stronger it reacts to photons. The main reason is that within a small energy range many states with different electronic configurations are available as opposed to a strongly bound (and hence energetically separated) ground-state wave function. This indicates that under certain conditions coupling to the quantum vacuum of a cavity can indeed modify ground state properties.

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F.B. would like to thank Enrico Ronca and Christian Schäfer for stimulating and useful discussions. Additionally, F.B. would like to thank Wilhelm Bender for sharing his experience about nonlinear programming. This work was supported by the European Research Council (ERC-2015-AdG694097), the Cluster of Excellence ”Advanced Imaging of Matter”(AIM), Grupos Consolidados (IT1249-19) and SFB925. The Flatiron Institute is a division of the Simons Foundation.

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