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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.

A summary of the technical advances that are incorporated in the fourth major release of the Q-Chem quantum chemistry program is provided, covering approximately the last seven years. These include developments in density functional theory methods and algorithms, nuclear magnetic resonance (NMR) property evaluation, coupled cluster and perturbation theories, methods for electronically excited and open-shell species, tools for treating extended environments, algorithms for walking on potential surfaces, analysis tools, energy and electron transfer modelling, parallel computing capabilities, and graphical user interfaces. In addition, a selection of example case studies that illustrate these capabilities is given. These include extensive benchmarks of the comparative accuracy of modern density functionals for bonded and non-bonded interactions, tests of attenuated second order Møller-Plesset (MP2) methods for intermolecular interactions, a variety of parallel performance benchmarks, and tests of the accuracy of implicit solvation models. Some specific chemical examples include calculations on the strongly correlated Cr2 dimer, exploring zeolite-catalysed ethane dehydrogenation, energy decomposition analysis of a charged ter-molecular complex arising from glycerol photoionisation, and natural transition orbitals for a Frenkel exciton state in a nine-unit model of a self-assembling nanotube.

Byline: Troy Van Voorhis; Tim Kowalczyk; Benjamin Kaduk; Lee-Ping Wang; Chiao-Lun Cheng; Qin Wu, Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139; email: tvan@mit.edu Keywords: reaction dynamics, nonadiabatic, density functional Author Notes: Present Address: Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973 Abstract Diabatic states have a long history in chemistry, beginning with early valence bond pictures of molecular bonding and extending through the construction of model potential energy surfaces to the modern proliferation of methods for computing these elusive states. In this review, we summarize the basic principles that define the diabatic basis and demonstrate how they can be applied in the specific context of constrained density functional theory. Using illustrative examples from electron transfer and chemical reactions, we show how the diabatic picture can be used to extract qualitative insight and quantitative predictions about energy landscapes. The review closes with a brief summary of the challenges and prospects for the further application of diabatic states in chemistry.

The constrained density functional theory-configuration interaction (CDFT-CI) method has previously been used to calculate ground-state energies and barrier heights, and to describe electronic excited states, in particular conical intersections. However, the method has been limited to evaluating the electronic energy at just a single nuclear configuration, with the gradient of the energy being available only via finite difference. In this paper, we present analytic gradients of the CDFT-CI energy with respect to nuclear coordinates, which gives the potential for accurate geometry optimization and molecular dynamics on both the ground and excited electronic states, a realm which is currently quite challenging for electronic structure theory. We report the performance of CDFT-CI geometry optimization for representative reaction transition states as well as molecules in an excited state. The overall accuracy of CDFT-CI for computing barrier heights is essentially unchanged whether the energies are evaluated at geometries obtained from quadratic configuration-interaction singles and doubles (QCISD) or CDFT-CI, indicating that CDFT-CI produces very good reaction transition states. These results open up tantalizing possibilities for future work on excited states. [ABSTRACT FROM AUTHOR]

The constrained density functional theory–configuration interaction (CDFT-CI) method has previously been used to calculate ground-state energies and barrier heights. In this work, it is examined for use in computing electronic excited states, for the challenging case of conical intersections. Conical intersections are a prevalent feature of excited electronic surfaces, but conventional time-dependent density functional theory calculations are found to be entirely unsatisfactory at describing them, for two small systems. CDFT-CI calculations on those systems are found to be in qualitative agreement with reference CAS surfaces. These results suggest that with a suitable definition of atomic populations and a careful choice of constrained states, CDFT-CI could be the basis for a seamless description of electronic degeneracy. [ABSTRACT FROM AUTHOR]

The full multiple spawning (FMS) method has been developed to simulate quantum dynamics in the multistate electronic problem. In FMS, the nuclear wave function is represented in a basis of coupled, frozen Gaussians, and a “spawning” procedure prescribes a means of adaptively increasing the size of this basis in order to capture population transfer between electronic states. Herein we detail a new algorithm for specifying the initial conditions of newly spawned basis functions that minimizes the number of spawned basis functions needed for convergence. “Optimally” spawned basis functions are placed to maximize the coupling between parent and child trajectories at the point of spawning. The method is tested with a two-state, one-mode avoided crossing model and a two-state, two-mode conical intersection model. [ABSTRACT FROM AUTHOR]

In this work, a constrained density functional theory based configuration interaction approach (CDFT-CI) is applied to calculating transition state energies of chemical reactions that involve bond forming and breaking at the same time. At a given point along the reaction path, the configuration space is spanned by two diabaticlike configurations: reactant and product. Each configuration is constructed self-consistently with spin and charge constraints to maximally retain the identities of the reactants or the products. Finally, the total energy is obtained by diagonalizing an effective Hamiltonian constructed in the basis spanned by these two configurations. By design, this prescription does not affect the energies of the reactant or product species but will affect the energy at intermediate points along the reaction coordinate, most notably by modifying the reaction barrier height. When tested with a large set of reactions that include hydrogen transfer, heavy atom transfer, and nucleophilic substitution, CDFT-CI is found to improve the prediction of barrier heights by a factor of 2–3 for some commonly used local, semilocal, and hybrid functionals. Thus, just as CDFT can be used to cure energy errors in charge localized states, CDFT-CI can recover the correct energy for charge delocalized states by approximating the true wave function as a linear combination of localized configurations (e.g., reactant and product). The well-defined procedure and the promising results of CDFT-CI suggest that it could broaden the applicability of traditional DFT methods for reaction barrier heights. [ABSTRACT FROM AUTHOR]

The article offers information on the constrained density functional theory (CDFT) for practical calculations. It states that the development of modern CDFT were benefited through the original presentation foresight of CDFT, in the manner of its applications and formalism. It mentions that analytical gradients were implemented for CDFT to enable ab initio molecular dynamics in the state of charge constrained of Marcus electron transfer theory.

Diabatic states have a long history in chemistry, beginning with early valence bond pictures of molecular bonding and extending through the construction of model potential energy surfaces to the modern proliferation of methods for computing these elusive states. In this review, we summarize the basic principles that define the diabatic basis and demonstrate how they can be applied in the specific context of constrained density functional theory. Using illustrative examples from electron transfer and chemical reactions, we show how the diabatic picture can be used to extract qualitative insight and quantitative predictions about energy landscapes. The review closes with a brief summary of the challenges and prospects for the further application of diabatic states in chemistry. [ABSTRACT FROM AUTHOR]