A mathematical analysis of the discretized IPT-DMFT equations

Abstract

In a previous contribution (E. Cancès, A. Kirsch and S. Perrin--Roussel, arXiv:2406.03384), we have proven the existence of a solution to the Dynamical Mean-Field Theory (DMFT) equations under the Iterated Perturbation Theory (IPT-DMFT) approximation. In view of numerical simulations, these equations need to be discretized. In this article, we are interested in a discretization of the \acrshort{ipt}-\acrshort{dmft} functional equations, based on the restriction of the hybridization function and local self-energy to a finite number of points in the upper half-plane $\left(iω_n\right)_{n \in |[0,N_ω]|}$, where $ω_n=(2n+1)π/ β$ is the $n$-th Matsubara frequency and $N_ω\in \mathbb N$. We first prove the existence of solutions to the discretized equations in some parameter range depending on $N_ω$. We then prove uniqueness for a smaller range of parameters. We also study more in depth the case of bipartite systems exhibiting particle-hole symmetry. In this case, the discretized IPT-DMFT equations have purely imaginary solutions, which can be obtained by solving a real algebraic system of $(N_ω+1)$ equations with $(N_ω+1)$ variables. We provide a complete characterization of the solutions for $N_ω=0$ and some results for $N_ω=1$ in the simple case of the Hubbard dimer. We finally present some numerical simulations on the Hubbard dimer.