Are multi-quasiparticle interactions important in molecular ionization?

Abstract

Photo-emission spectroscopy directly probes individual electronic states, ranging from single excitations to high-energy satellites, which simultaneously represent multiple quasiparticles (QPs) and encode information about electronic correlation. The first-principles description of the spectra requires an efficient and accurate treatment of all many-body effects. This is especially challenging for inner valence excitations where the single QP picture breaks down. Here, we provide the full valence spectra of small closed-shell molecules, exploring the independent and interacting quasiparticle regimes, computed with the fully correlated adaptive sampling configuration interaction method. We critically compare these results to calculations with the many-body perturbation theory, based on the GW and vertex corrected GWΓ approaches. The latter explicitly accounts for two-QP quantum interactions, which have often been neglected. We demonstrate that for molecular systems, the vertex correction universally improves the theoretical spectra, and it is crucial for the accurate prediction of QPs as well as capturing the rich satellite structures of high-energy excitations. GWΓ offers a unified description across all relevant energy scales. Our results suggest that the multi-QP regime corresponds to dynamical correlations, which can be described via perturbation theory.

Self-consistency in $GWΓ$ formalism leading to quasiparticle-quasiparticle couplings

“…As we will show next, this trend is repeated at all levels of perturbation theory: the K G kernel, and only this kernel, affects the satellites, leaving the main QP features intact. While for systems with more general interaction terms one would expect some effect on the main QP energies, these findings explain the numerical observations made previously [28,30,36,37].…”

“…In practice, K G introduces additional exchange coupling between the virtual particle-hole (ph) pairs and accounts for excitonic effects. Further, it partly corrects the self-polarization error [28,30,36,37].…”

Efficient treatment of molecular excitations in the liquid phase environment via stochastic many-body theory

“…43,75,76 Nevertheless, the GW correlation neglects the quantum fluctuations, which may become important for high energy excitations and/or for unoccupied states. 64,77 The method of subspace separations is, however, general and applicable to beyond-GW approaches; it will be explored in the future. Here, a single-shot perturbative correction is computed within the random phase approximation (RPA) on top of a density functional theory (DFT) starting point (see the SI for details).…”

“…To overcome the high cost of conventional implementations, we employ the recently developed stochastic formalism, which can be applied to extremely large systems owing to its linear scaling. 56,[58][59][60][61][62][63][64]77 This approach to the oneshot perturbative correction (typically denoted G 0 W 0 ) seeks the QP energy by randomly sampling the single-particle states and decomposing the operators in the real-time domain. The stochastic formalism has been described in detail in previous work [61][62][63] and it is briefly revisited in the SI.…”

“…This is particularly important for calculations that include higher-level treatment beyond GW . 64,77…”