This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design.

The electronically excited states of methylene (CH), ethylene (CH), butadiene (CH), hexatriene (CH), and ozone (O) have long proven challenging due to their complex mixtures of static and dynamic correlations. The semistochastic heat-bath configuration interaction (SHCI) algorithm, 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-ζ basis (ANO-L-pVDZ), which gives rise to a Hilbert space containing more than 10 determinants. These calculations give vertical excitation energies of 5.58 and 5.59 eV, respectively, for the 2A and 1B 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.

Tetracyanoquinodimethane bithiophene (QOT2) has a long-lived (57 μs) photoinduced excited state that may correspond to triplets resulting from intramolecular singlet fission (SF). Since SF usually occurs through intermolecular processes, a detailed description of the excited states involved and their evolution is needed to verify this hypothesis. The photoresponse of QOT2 is investigated using high-level electronic structure methods and quantum dynamics simulations, which show ultrafast passage through a conical intersection from the bright 1 1 B u state to the dark 2 1 A g surface. Characterization of QOT2's 2 1 A g wave function found it to be composed of two strongly coupled triplets, leading to the first detailed electronic structure description of an intramolecular 1 (TT) state. The population of such a state upon excitation of QOT2 raises the possibility of SF through conformational changes that decouple the triplets. However, reaching an appropriate geometry for decoupled triplets appears unlikely given the energy cost of 1.76 eV. Consequently, the hypothesis that the long-lived excited state corresponds to 2 1 A g , a spin singlet, strongly interacting double triplet, was explored. Transition moment calculations to assign excited-state absorption signals and investigations into internal conversion and intersystem crossing decay pathways indicate that a long-lived 2 1 A g state with 1 (TT) character is consistent with the available experimental data.

Difluoroboron b-diketonate (BF 2 bdk) dyes display reversible mechanochromic luminescence (ML) in the solid state. A series of BF 2 bdk dyes with methyl, phenyl, naphthyl and anthracyl groups (i.e. arene substituents and bdk ligands: Me-Ph ¼ mbm, Ph-Ph ¼ dbm, Np-Ph ¼ nbm, An-Ph ¼ abm) were prepared to test the effects of p conjugation length and arene size on ML properties. Solid-state emission spectra were recorded for powders, spin-cast films, and dye coated weighing paper. The materials emit at various wavelengths from blue to red, depending on the conjugation length. Additionally, emission spectra were recorded for smeared solids and their recovery was tracked at room temperature over time. All dyes except for BF 2 mbm show emission changes upon mechanical perturbation, with increasing p conjugation correlating with more dramatic, redshifted fluorescence, however, their recovery is significantly affected by the aromatic substituents. The thermal and structural properties of the dyes in the solid state were also investigated by differential scanning calorimetry (DSC) and atomic force microscopy (AFM).