Many-body effective energy theory: photoemission at strong correlation

Photoemission is a powerful tool to obtain insight into the electronic structure and excitations
in materials. From the theoretical point of viewMany-Body Perturbation Theory, within the so
called GW approximation to electron correlation, is the method of choice for calculations of
photoemission spectra of many materials. However GW suffers from some fundamental
shortcomings, and, in particular, it does not capture strong correlation, unless one treats the
system in a magnetically ordered phase. In this talk we illustrate some of these problems and
efforts to go overcome them [1-3]. In particular, we focus on a many-body effective-energy
theory (MEET) that gives many-body spectral functions in terms of reduced density matrices
(RDMs) [4]. We show that simple approximations, which require the knowledge of the lowest
n-body RDMs only, can provide accurate photoemission spectra in model systems in the weak
as well as strong correlation regime. With the example of several transition metal oxides, we
show that our method yields a qualitatively correct picture both in the antiferromagnetic and
paramagnetic phases, contrary to mean-field methods, in which the paramagnet is a metal
[4,5].

[1] P. Romaniello, F. Bechstedt, and L. Reining, Phys. Rev. B 85, 155131 (2012).
[2] A. Stan, P. Romaniello, S Rigamonti, L. Reining, and J.A. Berger, New J. Phys. 17, 093045
(2015).
[3] Pierre-François Loos, P. Romaniello, and J. A. Berger, J. Chem. Theory Comput. 14,
3071(2018); M. Véril, P. Romaniello, J. A. Berger, and Pierre-François Loos, J. Chem. Theory
Comput. 14, 5220 (2018).
[4] S. Di Sabatino, J.A. Berger, L. Reining, and P. Romaniello, Phys. Rev. B 94, 155141 (2016).
[5] S. Di Sabatino, J. A. Berger, and P. Romaniello, J. Chem. Theory Comput. 15, 5080 (2019)