Fate of the open-shell singlet ground state in the experimentally accessible acenes: a quantum Monte Carlo study

TL;DR

This study uses advanced quantum Monte Carlo methods to investigate the ground state properties of oligoacenes, revealing that long-range RVB correlations suppress open-shell diradical tendencies and suggest quasi-metallic behavior in larger acenes.

Contribution

It demonstrates that the RVB wave function provides a more accurate description than open-shell singlet wave functions for long acenes, challenging previous lower-level theories.

Findings

RVB correlations prevent diradical instabilities in acenes.

RVB wave functions have lower energy and better nodes than OSS.

Large acenes show quasi-metallic charge correlations.

Abstract

By means of the Jastrow correlated antisymmetrized geminal power (JAGP) wave function and quantum Monte Carlo (QMC) methods, we study the ground state properties of the oligoacene series, up to the nonacene. The JAGP is the accurate variational realization of the resonating-valence-bond (RVB) ansatz proposed by Pauling and Wheland to describe aromatic compounds. We show that the long-ranged RVB correlations built in the acenes' ground state are detrimental for the occurrence of open-shell diradical or polyradical instabilities, previously found by lower-level theories. We substantiate our outcome by a direct comparison with another wave function, tailored to be an open-shell singlet (OSS) for long-enough acenes. By comparing on the same footing the RVB and OSS wave functions, both optimized at a variational QMC level, and further projected by the lattice regularized diffusion Monte Carlo (LRDMC) method, we prove that the RVB wave function has always a lower variational energy and better nodes than the OSS, for all molecular species considered in this work. The entangled multi-reference RVB state acts against the electron edge localization implied by the OSS wave function, and weakens the diradical tendency for higher oligoacenes. These properties are reflected by several descriptors, including wave function parameters, bond length alternation, aromatic indices, and spin-spin correlation functions. In this context, we propose a new aromatic index estimator suitable for geminal wave functions. For the largest acenes taken into account, the long-range decay of the charge-charge correlation functions is compatible with a quasi-metallic behavior.