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.

The results of a systematic study of molecular properties by density functional theory (DFT) are presented and discussed. Equilibrium geometries, dipole moments, harmonic vibrational frequencies, and atomization energies were calculated for a set of 32 small neutral molecules by six different local and gradient-corrected DFT methods, and also by the ab initio methods Hartree-Fock, second-order Moller-Plesset, and quadratic configuration interaction with single and double substitutions (QCISD). The standard 6-31G* basis set was used for orbital expansion, and self-consistent Kohn-Sham orbitals were obtained by all DFT methods, without employing any auxiliary fitting techniques. Comparison with experimental results shows the density functional geometries and dipole moments to be generally no better than or inferior to those predicted by the conventional ab initio methods with this particular basis set. The density functional vibrational frequencies compare favorably with the ab initio results, while for atomization energies, two of the DFT methods give excellent agreement with experiment and are clearly superior to all other methods considered.

We present a simple algorithm, which we call the maximum overlap method (MOM), for finding excitedstate solutions to self-consistent field (SCF) equations. Instead of using the aufbau principle, the algorithm maximizes the overlap between the occupied orbitals on successive SCF iterations. This prevents variational collapse to the ground state and guides the SCF process toward the nearest, rather than the lowest energy, solution. The resulting excited-state solutions can be treated in the same way as the ground-state solution and, in particular, derivatives of excited-state energies can be computed using ground-state code. We assess the performance of our method by applying it to a variety of excited-state problems including the calculation of excitation energies, charge-transfer states, and excited-state properties.

The performance of a recently introduced hybrid of density functional theory and Hartree-Fock theory, the B-LYP/HF procedure, has been examined with a variety of basis sets. We have found that even the relatively small 6-31G* basis set yields atomization energies, ionization potentials and proton affinities whose mean absolute error, compared with a large body of accurate experimental data, is only 6.45 kcal/mol. We have also found that the addition of a "higher-level correction" (of the type used in G2 theory) to the B-LYP/HF total energies reduces the mean absolute error to 4.14 kcal/mol.

The accuracy of core excitation energies and core electron binding energies computed within a Delta self-consistent-field framework is assessed. The variational collapse of the core excited state is prevented by maintaining a singly occupied core orbital using an overlap criterion called the maximum overlap method. When applied to a wide range of small organic molecules, the resulting core excitation energies are not systematically underestimated as observed in time-dependent density functional theory and agree well with experiment. The accuracy of this approach for core excited states is illustrated by the calculation of the pre-edge features in x-ray absorption spectra of plastocyanin, which shows that accurate results can be achieved with Delta self-consistent-field calculations when used in conjunction with uncontracted basis functions.

An efficient and reasonably accurate grid, designated SG-1, is proposed for use in density functional calculations. Defined for all atoms from H to Ar, SC&I is recommended as a standard grid, analogous to the various standard basis sets which are used in contemporary quantum chemistry. In calculations on systems of moderate size, the differences between SC-I and very large grids are of the order of 0.2 kcal/mol, yet SG-1 is sufficiently small to be applied to large systems.

Q-Chem is a general-purpose electronic structure package featuring a variety of established and new methods implemented using innovative algorithms that enable fast calculations of large systems on regular laboratory workstations using density functional and wave-function-based approaches. It features an integrated graphical interface and input generator, a large selection of functionals and correlation approaches including methods for electronically excited states and openshell systems. In addition to serving the computational chemistry community, 'To understand something means to derive it from quantum mechanics that nobody understands.' -Anonymous Q uantum mechanics (QM) provides fundamental laws governing properties of matter on the atomic scale.Ĥ, the Hamiltonian, defines the system (the number of nuclei and electrons and their interactions with each other and external potentials), and , the wave function, has all the answers. By solving the Schrödinger equation,Ĥ = E , one can find equilibrium structures of molecules and materials, compute all sorts of spectra, and calculate thermochemical quantities that determine reaction rates and yields. Thus, as eloquently pointed out by Dirac in 1929, the laws determining all of chemistry (and a large part of physics) are completely known. Yet, the practical application of these laws is limited by the computationally demanding nature of the underlying equations. Developing approximate practical methods for applying QM to describe matter is what defines the field of quantum chemistry. Advances in computer technology, together with progress in developing efficient approximate methods and computer codes for solving the Schrödinger equation, have made quantum chem- * Correspondence