Multireference Algebraic Diagrammatic Construction Theory for Excited States: Extended Second-Order Implementation and Benchmark

TL;DR

This paper introduces extended second-order multireference algebraic diagrammatic construction (MR-ADC) methods for excited state simulations, demonstrating improved accuracy and efficiency over existing methods in strongly correlated molecular systems.

Contribution

The paper develops and benchmarks the strict and extended second-order MR-ADC methods, enhancing excited state calculations without relying on state-averaged references.

Findings

MR-ADC(2) and MR-ADC(2)-X outperform third-order single-reference ADC for weakly correlated states.

MR-ADC methods are competitive with EOM-CC methods for certain excited states.

They offer straightforward, efficient calculations and access to non-frontier excitations.

Abstract

We present an implementation and benchmark of new approximations in multireference algebraic diagrammatic construction theory for simulations of neutral electronic excitations and UV/Vis spectra of strongly correlated molecular systems (MR-ADC). Following our work on the first-order MR-ADC approximation [J. Chem. Phys. 2018, 149, 204113], we report the strict and extended second-order MR-ADC methods (MR-ADC(2) and MR-ADC(2)-X) that combine the description of static and dynamic electron correlation in the ground and excited electronic states without relying on state-averaged reference wavefunctions. We present an extensive benchmark of the new MR-ADC methods for excited states in several small molecules, including the carbon dimer, ethylene, and butadiene. Our results demonstrate that for weakly-correlated electronic states the MR-ADC(2) and MR-ADC(2)-X methods outperform the third-order single-reference ADC approximation and are competitive with the results from equation-of-motion coupled cluster theory. For states with multireference character, the performance of the MR-ADC methods is similar to that of an N-electron valence perturbation theory. In contrast to conventional multireference perturbation theories, the MR-ADC methods have a number of attractive features, such as a straightforward and efficient calculation of excited-state properties and a direct access to excitations outside of the frontier (active) orbitals.