Approaching Coupled Cluster Accuracy with Positive Semidefinite Vertex Corrected Self-Energies

Hedin's formalism of functional derivatives is a successful method for systematically constructing correlated electronic theories, largely due to the success of its lowest-order self-energy expansion, the GW approximation. Beyond GW, diagrammatic resummation schemes attempt to mix correlations simultaneously across all particle-particle and particle-hole channels. Because such a comprehensive treatment is computationally prohibitive for realistic molecular systems, a highly effective alternative is to fully account for electronic correlations in one specific channel, typically the particle-hole channel. This idea was recently implemented for molecular systems, yielding a self-energy expressed in terms of excited-state energies and transition amplitudes from the solution of the Bethe-Salpeter equation, rather than the random-phase approximation used in GW. While this approach is predictive and numerically efficient, it violates the fundamental positive-definiteness constraint on the electron spectral function in certain energy ranges. In this study, we resolve this physical flaw by deriving a positive-semidefinite (PSD) extension of the theory using a rigorous framework based on the nonequilibrium Green's function formalism. The PSD constraint introduces new scattering channels and triplet intermediate states and restores the correct physical behavior. We demonstrate that it consistently improves quasiparticle energies across standard molecular benchmarks, with an accuracy comparable to coupled-cluster reference calculations.