Orbital Optimization in the Density Matrix Renormalization Group, with applications to polyenes and \beta-carotene

TL;DR

This paper introduces a DMRG-based orbital optimization method, enabling highly accurate calculations of excited states in large polyenes and arotene, advancing computational quantum chemistry techniques.

Contribution

The paper presents a novel DMRG-CASSCF approach that allows self-consistent orbital optimization for large active spaces, improving accuracy over traditional methods.

Findings

Accurately computed low-lying excited states of polyenes up to C24H26.

Correlated arotene's arotene's spectra with experimental resonance Raman data.

Revealed new optically dark states in polyenes.

Abstract

In previous work we have shown that the Density Matrix Renormalization Group (DMRG) enables near-exact calculations in active spaces much larger than are possible with traditional Complete Active Space algorithms. Here, we implement orbital optimisation with the Density Matrix Renormalization Group to further allow the self-consistent improvement of the active orbitals, as is done in the Complete Active Space Self-Consistent Field (CASSCF) method. We use our resulting DMRGCASSCF method to study the low-lying excited states of the all-trans polyenes up to C24H26 as well as \beta-carotene, correlating with near-exact accuracy the optimised complete \pi-valence space with up to 24 active electrons and orbitals, and analyse our results in the light of the recent discovery from Resonance Raman experiments of new optically dark states in the spectrum.