PEXSI-$\Sigma$: A Green's function embedding method for Kohn-Sham density functional theory

TL;DR

The paper introduces PEXSI-$ Sigma$, a new Green's function embedding method for Kohn-Sham DFT that achieves near-linear scaling and accurately models complex systems with boundary self-energy matrices.

Contribution

It presents a novel Green's function embedding approach, PEXSI-$ Sigma$, that efficiently approximates the density matrix with boundary self-energy matrices, enabling scalable KSDFT calculations.

Findings

Accurately models graphene defects using PEXSI-$ Sigma$

Achieves $\\mathcal{O}(N^2)$ computational complexity

Demonstrates compatibility with atomistic Green's function methods

Abstract

In this paper, we propose a new Green's function embedding method called PEXSI-$\Sigma$ for describing complex systems within the Kohn-Sham density functional theory (KSDFT) framework, after revisiting the physics literature of Green's function embedding methods from a numerical linear algebra perspective. The PEXSI-$\Sigma$ method approximates the density matrix using a set of nearly optimally chosen Green's functions evaluated at complex frequencies. For each Green's function, the complex boundary conditions are described by a self energy matrix $\Sigma$ constructed from a physical reference Green's function, which can be computed relatively easily. In the linear regime, such treatment of the boundary condition can be numerically exact. The support of the $\Sigma$ matrix is restricted to degrees of freedom near the boundary of computational domain, and can be interpreted as a frequency dependent surface potential. This makes it possible to perform KSDFT calculations with $\mathcal{O}(N^2)$ computational complexity, where $N$ is the number of atoms within the computational domain. Green's function embedding methods are also naturally compatible with atomistic Green's function methods for relaxing the atomic configuration outside the computational domain. As a proof of concept, we demonstrate the accuracy of the PEXSI-$\Sigma$ method for graphene with divacancy and dislocation dipole type of defects using the DFTB+ software package.