Total energies from variational functionals of the Green function and the renormalized four-point vertex

TL;DR

This paper develops variational functionals for total energy calculations in many-body systems using Green functions and four-point vertices, ensuring conservation laws and enabling accurate, low-cost approximations for highly correlated systems.

Contribution

It introduces a new variational functional framework based on the $ ext{Xi}$-functional that guarantees conservation laws and allows for efficient total energy computations from approximate Green functions and vertices.

Findings

Functional guarantees second-order error in total energy

Applicable to highly correlated systems with ladder and exchange diagrams

Provides a practical computational scheme for accurate energies

Abstract

We derive variational expressions for the grand potential or action in terms of the many-body Green function $G$ which describes the propagation of particles and the renormalized four-point vertex $\Gamma$ which describes the scattering of two particles in many-body systems. The main ingredient of the variational functionals is a term we denote as the $\Xi$-functional which plays a role analogously to the usual $\Phi$-functional studied by Baym (G.Baym, Phys.Rev. 127, 1391 (1962)) in connection with the conservation laws in many-body systems. We show that any $\Xi$-derivable theory is also $\Phi$-derivable and therefore respects the conservation laws. We further set up a computational scheme to obtain accurate total energies from our variational functionals without having to solve computationally expensive sets of self-consistent equations. The input of the functional is an approximate Green function $\tilde{G}$ and an approximate four-point vertex $\tilde{\Gamma}$ obtained at a relatively low computational cost. The variational property of the functional guarantees that the error in the total energy is only of second order in deviations of the input Green function and vertex from the self-consistent ones that make the functional stationary. The functionals that we will consider for practical applications correspond to infinite order summations of ladder and exchange diagrams and are therefore particularly suited for applications to highly correlated systems. Their practical evaluation is discussed in detail.