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.

Utilizing plasmon-generated hot carriers to drive chemical reactions has currently become an active area of research in solar photocatalysis at the nanoscale. However, the mechanism underlying exact transfer and the...

The mechanism and selectivity of the asymmetric Friedel-Crafts (F-C) alkylation reaction between indole and chalcone catalyzed by chiral N, N'-dioxide-Sc(III) complexes were investigated at the M06/6-311+G(d,p)//M06/[LANL2DZ,6-31G(d)](SMD,CHCl) level. The reaction occurred via a three-step mechanism: (i) the C-C bond formation by interacting the most mucleophilic C center of indole with the most electrophilic C center of chalcone; (ii) the abstraction of the proton at the C atom of indole by counterion OTf; (iii) proton transfer from HOTf to the C atom of chalcone, generating the F-C alkylation product. The reaction preferred to occur along the favorable re-face attack pathway, producing the dominant R-product. The turnover frequency (TOF) of catalysis was predicted to be 1.59 × 10 s, with a rate constant of K( T) = 1.58 × 10 exp(-29057/ RT) dm·mol·s over the temperature range of 248-368 K. Activation strain model (ASM) and energy decomposition analysis (EDA), as well as noncovalent interaction (NCI) analysis, for the stereocontrolling transition state revealed that the substituent attached to the N atom of the amide subunits as well as the amino acid backbone of ligand played important roles in chiral inductivity. The benzyl group with structural flexibility tended to form strong π-π stacking with substrate as well as the terminal phenyl group of chalcone, stabilizing re-face attack transition state.

A new diabatization scheme is proposed to calculate the electronic couplings for the singlet fission process in multichromophoric systems. In this approach, a robust descriptor that treats single and multiple excitations on an equal footing is adopted to quantify the localization degree of the particle and hole densities of the electronic states. By maximally localizing the particles and holes in terms of predefined molecular fragments, quasi-diabatic states with well-defined characters (locally excited, charge transfer, correlated triplet pair, etc.) can be automatically constructed as the linear combinations of the adiabatic ones, and the electronic couplings can be directly obtained. This approach is very general in that it applies to electronic states with various spin multiplicities and can be combined with various kinds of preliminary electronic structure calculations. Due to the high numerical efficiency, it is able to manipulate more than 100 electronic states in diabatization. The applications to the tetracene dimer and trimer reveal that high-lying multiply excited charge transfer states have significant influences on both the formation and separation of the correlated triplet pair and can even enlarge the coupling for the latter process by 1 order of magnitude.

The nonadiabatic phenomena, which are characterized by a strong coupling between electronic and nuclear motions, are ubiquitous. The nonadiabatic effect of the studied system can be significantly affected by the surrounding environment such as solvents, in which such nonadiabatic process takes place. The time-dependent density functional theory (TDDFT) is currently the most efficient approach to describe the electronic structures and dynamics of complex systems, while the polarizable continuum model (PCM) represents one of the most successful examples among continuum solvation models. Here, we formulate the first-order derivative couplings (DCs) between the ground and excited states as well as between two excited states by utilising time-independent equation of motion (EOM) formalism within the framework of both linear response and spin flip formulations of TDDFT/CPCM (the conductor-like PCM), and implement the analytical approach into the Q-Chem electronic structure software package. The analytic implementation is validated by the comparison of the analytical and finite-difference results, reproducing geometric phase effect in the protonated formaldimine test case. Taking 4-(\textit{N},\textit{N}-dimethylamino)benzonitrile and uracil in gas phase and solution as an example, we demonstrate that the solvent effect is essential not only for the excitation energies of the low-lying excited-states, but also for the DCs between these states. Finally, we calculate the internal conversion rate of benzophenone in solvent with DC being used. The current implementation of analytical DCs together with the existing analytical gradient and Hessian of TDDFT/PCM excited states allows one to study the nonadiabatic effects of relatively large systems in solutions with low computational cost.

A new diabatization scheme is proposed to calculate the electronic couplings for the singlet fission process in multichromophoric systems. In this approach, a robust descriptor that treats single and multiple excitations on an equal footing is adopted to quantify the localization degree of the particle and hole densities of the electronic states. By maximally localizing the particles and holes in terms of predefined molecular fragments, quasi-diabatic states with well-defined characters (locally excited, charge transfer, correlated triplet pair, etc.) can be automatically constructed as the linear combinations of the adiabatic ones, and the electronic couplings can be directly obtained. This approach is very general in that it applies to electronic states with various spin multiplicities and can be combined with various kinds of preliminary electronic structure calculations. Due to the high numerical efficiency, it is able to manipulate more than 100 electronic states in diabatization. The applications to the tetracene dimer and trimer reveals that high-lying multiply-excited charge transfer states have significant influences on both the formation and separation of the correlated triplet pair, and can even enlarge the coupling for the latter process by one order of magnitude.

A new diabatization scheme is proposed to calculate the electronic couplings for the singlet fission process in multichromophoric systems. In this approach, a robust descriptor that treats single and multiple excitations on an equal footing is adopted to quantify the localization degree of the particle and hole densities of the electronic states. By maximally localizing the particles and holes in terms of predefined molecular fragments, quasi-diabatic states with well-defined characters (locally excited, charge transfer, correlated triplet pair, etc.) can be automatically constructed as the linear combinations of the adiabatic ones, and the electronic couplings can be directly obtained. This approach is very general in that it applies to electronic states with various spin multiplicities and can be combined with various kinds of preliminary electronic structure calculations. Due to the high numerical efficiency, it is able to manipulate more than 100 electronic states in diabatization. The applications to the tetracene dimer and trimer reveals that high-lying multiply-excited charge transfer states have significant influences on both the formation and separation of the correlated triplet pair, and can even enlarge the coupling for the latter process by one order of magnitude.

Utilizing plasmon-generated hot carriers to drive chemical reactions has currently become an active area of research in solar photocatalysis at the nanoscale. However, the mechanism underlying exact transfer and the generation dynamics of hot carriers, and the strategies used to further improve the quantum efficiency of the photocatalytic reaction still deserve a further look. In this work, we perform a nonadiabatic excited-state dynamics study to depict the correlation between the reaction rate of plasmon-driven water splitting (PDWS) and the sizes of gold particles, the incident light frequency and intensity, and the near-field's spatial distribution. Four model systems, \ce{H2O} and \ce{Au20}@\ce{H2O} separately interacting with the laser field and the near field generated by the Au nanoparticle (NP) with a few nanometers in size, have been investigated. Our simulated results clearly unveil the mechanism of PDWS and hot-electron injection in a Schottky-free junction: the electrons populated on the antibonding orbitals of \ce{H2O} are mandatory to drive the \ce{OH} bond breaking and the strong orbital hybridization between \ce{Au20} and \ce{H2O} creates the condition for direct electron injection. We further find that the linear dependence of the reaction rate and the field amplitude only holds at a relatively weak field and it breaks down when the second {\ce{OH}} bond begins to dissociate and field-induced water fragmenting at a very intensive field, and that with the guarantee of electron injection, the water splitting rate increases with the increase of NP's size. This study will be helpful for further improving the efficiency of the photochemical reactions involving the plasmon-generated hot carriers and expanding the applications of hot carriers in varieties of chemical reactions.