Reference excitation energies of increasingly large molecules: a QMC study of cyanine dyes

TL;DR

This study uses advanced quantum Monte Carlo methods to accurately compute excitation energies of cyanine dyes, demonstrating scalability to larger molecules and identifying key factors for precise excited-state descriptions.

Contribution

The paper introduces a systematic protocol for constructing multi-determinant wave functions in QMC, enabling accurate excitation energy calculations for larger cyanine dyes beyond previous capabilities.

Findings

QMC results agree with high-level coupled cluster calculations for small cyanines.

The protocol is effective for longer chains, establishing QMC as a reference method for large systems.

Active $\pi$ orbitals alone are insufficient for accurate excitation descriptions in shorter chains.

Abstract

We revisit here the lowest vertical excitations of cyanine dyes using quantum Monte Carlo and leverage on recent developments to systematically improve on previous results. In particular, we employ a protocol for the construction of compact and accurate multi-determinant Jastrow-Slater wave functions for multiple states, which we have recently validated on the excited-state properties of several small prototypical molecules. Here, we obtain quantum Monte Carlo excitation energies in excellent agreement with high-level coupled cluster for all the cyanines where the coupled cluster method is applicable. Furthermore, we push our protocol to longer chains, demonstrating that quantum Monte Carlo is a viable methodology to establish reference data at system sizes which are hard to reach with other high-end approaches of similar accuracy. Finally, we determine which ingredients are key to an accurate treatment of these challenging systems and rationalize why a description of the excitation based on only active $\pi$ orbitals lacks the desired accuracy for the shorter chains.