A balanced treatment of static and dynamic correlation in free- and Mg-porphyrin

TL;DR

This paper introduces an ab-initio dynamical configuration interaction (DCI) method that combines wave function and Green's function approaches to accurately and efficiently describe static and dynamic correlation in large, strongly-correlated molecular systems like porphyrins.

Contribution

The paper develops and applies a novel DCI method that balances static and dynamic correlation, demonstrating its effectiveness on porphyrin systems with good agreement to experimental and benchmark data.

Findings

DCI accurately predicts excitation energies of porphyrins.

The method captures size-extensive correlation effects.

Good agreement with experimental and benchmark results.

Abstract

We present an ab-initio dynamical configuration interaction (DCI) study of free- and Mg-porphyrin. DCI is a recently developed active space theory based on the L\"owdin downfolding technique. In the active space, static correlation is described exactly with full configuration interaction. In the high energy, dynamically correlated subspace, we treat correlation at the quasiparticle level in the $GW$ approximation of Green's function theory. The final theory combines wave function and Green's function methods to give a balanced description of static and dynamic correlation. The theory and algorithm give a multireference treatment of ground and excited states for low computational cost. The four orbital Gouterman model of porphyrin offers an ideal active space in a large, correlated system to test the cost and accuracy of the embedding for large systems. Our parameter free, fully ab-initio DCI calculations in the minimal four-level active space agree well with both experiment and more expensive benchmark theories for the $Q_x$ and $Q_y$ transitions of free- and Mg-porphyrin. Studying the convergence of the excitation energies suggests that DCI correctly captures size extensive correlation effects, making it a promising active space theory for large, strongly-correlated systems.