Low rank compression in the numerical solution of the nonequilibrium Dyson equation

TL;DR

This paper introduces a hierarchical low rank compression method for the nonequilibrium Dyson equation, significantly reducing computational and memory costs, enabling longer simulations in quantum many-body physics.

Contribution

The paper presents a novel hierarchical low rank data structure and algorithm that accelerates and reduces memory usage in solving the nonequilibrium Dyson equation.

Findings

Reduces computational complexity from cubic to near quadratic.

Decreases memory complexity from quadratic to near linear.

Enables simulation of large systems with thousands of time steps in feasible time.

Abstract

We propose a method to improve the computational and memory efficiency of numerical solvers for the nonequilibrium Dyson equation in the Keldysh formalism. It is based on the empirical observation that the nonequilibrium Green's functions and self energies arising in many problems of physical interest, discretized as matrices, have low rank off-diagonal blocks, and can therefore be compressed using a hierarchical low rank data structure. We describe an efficient algorithm to build this compressed representation on the fly during the course of time stepping, and use the representation to reduce the cost of computing history integrals, which is the main computational bottleneck. For systems with the hierarchical low rank property, our method reduces the computational complexity of solving the nonequilibrium Dyson equation from cubic to near quadratic, and the memory complexity from quadratic to near linear. We demonstrate the full solver for the Falicov-Kimball model exposed to a rapid ramp and Floquet driving of system parameters, and are able to increase feasible propagation times substantially. We present examples with 262144 time steps, which would require approximately five months of computing time and 2.2 TB of memory using the direct time stepping method, but can be completed in just over a day on a laptop with less than 4 GB of memory using our method. We also confirm the hierarchical low rank property for the driven Hubbard model in the weak coupling regime within the GW approximation, and in the strong coupling regime within dynamical mean-field theory.