Excited States of Methylene, Polyenes, and Ozone from Heat-Bath Configuration Interaction

TL;DR

This paper employs semistochastic heat-bath configuration interaction to perform the largest FCI-level calculations on complex molecules like hexatriene and ozone, providing accurate excitation energies and supporting the existence of a ring-minimum ozone species.

Contribution

It introduces the largest FCI-level calculations for polyenes and ozone using SHCI, achieving near-complete correlation energy recovery and providing new insights into ozone's ring-minimum structure.

Findings

Excitation energies for hexatriene's states are nearly degenerate at 5.58 and 5.59 eV.

Calculated excitation energies for butadiene are 6.58 and 6.45 eV.

Strong support for a ring-minimum ozone species 1.3 eV above the open-ring form.

Abstract

The electronically excited states of methylene (CH$_2$), ethylene (C$_2$H$_4$), butadiene (C$_4$H$_6$), hexatriene (C$_6$H$_8$), and ozone (O$_3$) have long proven challenging due to their complex mixtures of static and dynamic correlations. Semistochastic heat-bath configuration interaction (SHCI), which efficiently and systematically approaches the full configuration interaction (FCI) limit, is used to provide close approximations to the FCI energies in these systems. This article presents the largest FCI-level calculation to date -- on hexatriene using a polarized double-zeta basis (ANO-L-pVDZ), which gives rise to a Hilbert space containing more than $10^{38}$ determinants. These calculations give vertical excitation energies of 5.58 and 5.59 eV respectively for the $2^1{\rm A}_{\rm g}$ and $1^1{\rm B}_{\rm u}$ states, showing that they are nearly degenerate. The same excitation energies in butadiene/ANO-L-pVDZ were found to be 6.58 and 6.45 eV. In addition to these benchmarks, our calculations strongly support the presence of a previously hypothesized ring-minimum species of ozone that lies 1.3 eV higher than the open-ring minimum energy structure and is separated from it by a barrier of 1.11 eV.