# Second-Order Multi-Reference Algebraic Diagrammatic Construction Theory for Photoelectron Spectra of Strongly Correlated Systems

## TL;DR

This paper introduces MR-ADC(2), a second-order multi-reference algebraic diagrammatic construction method that efficiently simulates photoelectron spectra of strongly correlated systems, capturing ionization energies and intensities across all orbitals.

## Contribution

The paper develops a new second-order MR-ADC(2) method that avoids large density matrices and offers lower computational scaling for photoelectron spectrum simulations.

## Key findings

- Successfully applied to small molecules, carbon dimer, and hydrogen chains.
- Provides comprehensive ionization information for all orbitals.
- Achieves lower computational cost compared to traditional methods.

## Abstract

We present a second-order formulation of multi-reference algebraic diagrammatic construction theory [Sokolov, A. Yu. J. Chem. Phys. 2018, 149, 204113] for simulating photoelectron spectra of strongly correlated systems (MR-ADC(2)). The MR-ADC(2) method uses second-order multi-reference perturbation theory (MRPT2) to efficiently obtain ionization energies and intensities for many photoelectron transitions in a single computation. In contrast to conventional MRPT2 methods, MR-ADC(2) provides information about ionization of electrons in all orbitals (i.e., core and active) and allows to compute transition intensities in straightforward and efficient way. Although equations of MR-ADC(2) depend on four-particle reduced density matrices, we demonstrate that computation of these large matrices can be completely avoided without introducing any approximations. The resulting MR-ADC(2) implementation has a lower computational scaling compared to conventional MRPT2 methods. We present results of MR-ADC(2) for photoelectron spectra of small molecules, carbon dimer, and equally-spaced hydrogen chains (H10 and H30) and outline directions for future developments.