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...

Advances in theory and algorithms for electronic structure calculations must be incorporated into program packages to enable them to become routinely used by the broader chemical community. This work reviews advances made over the past five years or so that constitute the major improvements contained in a new release of the Q-Chem quantum chemistry package, together with illustrative timings and applications. Specific developments discussed include fast methods for density functional theory calculations, linear scaling evaluation of energies, NMR chemical shifts and electric properties, fast auxiliary basis function methods for correlated energies and gradients, equation-of-motion coupled cluster methods for ground and excited states, geminal wavefunctions, embedding methods and techniques for exploring potential energy surfaces.

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This work considers calculation of pK a for a series of related alcohols, carboxylic acids, and ammonium ions spanning a wide range of acidities, using quantum mechanical treatment of solute electronic structure in conjunction with a dielectric continuum model for solvation of each bare solute. The electronic structure methods used are of sufficiently high quality to give very good agreement with experimental gas phase acidities. Dielectric continuum theory of solvation is used in a recently developed form that accurately takes account of solute charge density penetrating outside the solvent cavity that nominally encloses it. The cavity surface is defined by a single parameter characterizing an electronic isodensity contour, and contours are examined at and near the value 0.001 e/ that has previously led to a good account of solvation effects on properties of neutral solutes in various solvents. In water, the pK a values calculated for alcohols and carboxylic acids are generally much higher than experiment, while for ammonium ions they are comparable to experiment. Good results in water can be obtained from linear correlations that describe the effects of different substituents in solutes sharing the same acidic functional group, but different correlations apply for different acidic functional groups. For the polar nonprotic solvents DMSO and MeCN, pK a values close to experimental results are obtained, and very good linear correlations are found that simultaneously describe well all the solutes considered. It is argued this indicates that dielectric continuum theory properly accounts for long-range bulk solvent effects on pK a without the need for special parameterization of the cavity. To achieve good pK a results in water, further account must be taken of specific short-range effects such as hydrogen bonding. Rather than distort the cavity from the physical solute−solvent interface region in order to artificially force dielectric continuum theory to serve this purpose, as is commonly done through detailed parameterization schemes, it is recommended that other complementary approaches more appropriate for describing short-range interactions should be sought to complete the treatment of solvation effects in water.