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 openshell 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 Cr 2 dimer, exploring zeolitecatalysed 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.Keywords quantum chemistry, software, electronic structure theory, density functional theory, electron correlation, computational modelling, Q-Chem Disciplines Chemistry CommentsThis article is from Molecular Physics: An International Journal at the Interface Between Chemistry and Physics 113 (2015): 184, doi:10.1080/00268976.2014. RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. Authors 185A 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 corre...

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 production-level implementation of equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) for electron attachment and excitation energies augmented by a complex absorbing potential (CAP) is presented. The new method enables the treatment of metastable states within the EOM-CC formalism in a similar manner as bound states. The numeric performance of the method and the sensitivity of resonance positions and lifetimes to the CAP parameters and the choice of one-electron basis set are investigated. A protocol for studying molecular shape resonances based on the use of standard basis sets and a universal criterion for choosing the CAP parameters are presented. Our results for a variety of π(*) shape resonances of small to medium-size molecules demonstrate that CAP-augmented EOM-CCSD is competitive relative to other theoretical approaches for the treatment of resonances and is often able to reproduce experimental results.

We report the experimental observation of water dangling OH bonds in the hydration shells around dissolved nonpolar (hydrocarbon) groups. The results are obtained by combining vibrational (Raman) spectroscopy and multivariate curve resolution (MCR), to reveal a high-frequency OH stretch peak arising from the hydration shell around nonpolar (hydrocarbon) solute groups. The frequency and width of the observed peak is similar to that of dangling OH bonds previously detected at macroscopic air-water and oil-water interfaces. The area of the observed peak is used to quantify the number of water dangling bonds around hydrocarbon chains of different length. Molecular dynamics simulation of the vibrational spectra of water molecules in the hydration shell around neopentane and benzene reveals high-frequency OH features that closely resemble the experimentally observed dangling OH vibrational bands around neopentyl alcohol and benzyl alcohol. The red-shift of Ϸ50 cm ؊1 induced by aromatic solutes is similar to that previously observed upon formation of a -H bond (in low-temperature benzene-water clusters).hydrophobic ͉ interface ͉ vibration ͉ Raman C hanges in the structure and dynamics of water induced by nonpolar groups have long been considered to play a key role in protein folding, ligand binding, and the formation of biological cell membranes (1). Early thermodynamic evidence suggested that water may form an ''iceberg'' or clathrate-like structure around nonpolar molecules (2). Although no such rigid structures are currently thought to form (3), recent experimental (4, 5) and theoretical (6) results indicate that the rotational mobility of water molecules is reduced around nonpolar solutes (relative to bulk water). Moreover, fundamental theoretical arguments (and simulation measurements) imply that the size of a hydrophobic group may play a critical role in dictating water structure (7). This has led to the provocative suggestion that the structure of water around hydrophobic groups of nanometer (or greater) size may bear some resemblance to that at a macroscopic air-water interface (8). Although cohesive interactions between water and nonpolar groups tend to suppress the formation of an interfacial vapor layer (8-11), recent experimental (and molecular dynamics) studies indicate that the nonpolar binding cavity in bovine ␤-lactoglobulin is completely dehydrated in liquid water (12). Moreover, both experimental and simulation evidence suggests that water at nonpolar interfaces experiences significantly larger fluctuations than either bulk water or water at hydrophilic interfaces (13,14). Here, we present experimental evidence that reveals a similarity between the structure of water around dissolved hydrocarbon groups and that at macroscopic oil-water interfaces (15-18), in the sense that both interfaces induce the formation of dangling OH bonds.We have detected water dangling OH bonds by combining vibrational Raman spectroscopy with multivariate curve resolution (MCR). This procedure is used to decompose solution sp...

We propose a method for calculation of oxidation states in transition metal complexes, utilizing a bonding analysis based on localized molecular orbitals in conjunction with traditional population analyses. The localized orbital bonding analysis (LOBA) is seen to accurately produce both the oxidation state and chemically intuitive views of bonding in the complexes studied. This is in contrast to simple population analyses where the oxidation states are not reproduced for even simple systems and more complex analyses which break down on problematic systems. We report the application to a manganese complex with potential activity as oxygen-evolving catalyst, determining the location of the oxidations.