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.

Many applications in quantum information science (QIS) rely on the ability to laser-cool molecules. The scope of applications can be expanded if laser-coolable molecules possess two or more cycling centers, i.e., moieties capable of scattering photons via multiple absorption-emission events. Here we employ equation-of-motion coupled-cluster method for double electron attachment (EOM-DEA-CCSD) to study electronic structure of hypermetallic molecules with two alkaline earth metals connected by an acetylene linker. We demonstrate that the interaction between two unpaired electrons is weak yet non-negligible, and is reflected in the underlying wavefunction. The electronic structure of the molecules is similar to that of two separated alkali metals, however the interaction between two electrons is largely dominated by through-bond interactions. The communication between the two cycling centers is quantified by the extent of the entanglement of the two unpaired electrons associated with each center. This contribution highlights rich electronic structure of hypermetallic molecules that may advance various applications in QIS and beyond. File list (2) download file view on ChemRxiv mccm.pdf (2.25 MiB) download file view on ChemRxiv si.pdf (4.08 MiB)

Cyanopolyyne anions were among the first anions discovered in the interstellar medium. The discovery has raised questions about the routes of formation of these anions in space. Some of the proposed mechanisms assumed that anionic excited electronic states, either metastable or weakly bound, play a key role in the formation process. The verification of this hypothesis requires detailed knowledge of the electronic states of the anions. Here we investigate the bound and continuum states of four cyanopolyyne anions, CN, CN, CN, and CN, by means of ab initio calculations. We employ the equation-of-motion coupled-cluster method augmented with complex absorbing potential. We predict that already in CN, the smallest anion in the family, there are several low-lying metastable states of both singlet and triplet spin symmetry. These states, identified as shape resonances, are located between 6.3-8.5 eV above the ground state of the anion (or 2.3-4.5 eV above the ground state of the parent radical) and have widths of a few tenths of eV up to 1 eV. We analyze the identified resonances in terms of leading molecular orbital contributions and Dyson orbitals. As the carbon chain length increases in the CN series, these resonances gradually become stabilized and eventually turn into stable valence bound states. The trends in the energies of the transitions leading to both resonance and bound excited states can be rationalized by means of the Hückel model. Apart from valence excited states, some of the cyanopolyynes can also support dipole bound states and dipole stabilized resonances, owing to a large dipole moment of the parent radicals in the lowest Σ state.

Electronic structure calculations for C2, C2−, and C22− using the CC/EOM-CC family of methods. Results illustrate that EOM-CCSD provides an attractive alternative to MR approaches.