Shao Y.,Q-Chem, Inc. |
Gan Z.,Q-Chem, Inc. |
Epifanovsky E.,Q-Chem, Inc. |
Epifanovsky E.,University of Southern California |
And 187 more authors.
Molecular Physics | Year: 2015
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. © 2014 © 2014 Taylor & Francis.
Honmura Y.,Hirosaki University |
Takekawa H.,Hirosaki University |
Tanaka K.,Hirosaki University |
Maeda H.,Hirosaki University |
And 3 more authors.
Journal of Natural Products | Year: 2015
The structures of epoxyroussoenone (1) and epoxyroussoedione (3) isolated from a culture broth of Roussoella japanensis KT1651 were determined. Although NMR spectra provided insufficient structural information, computation of the theoretical chemical shifts with DFT EDF2/6-31G∗ enabled us to elucidate not only the planar structure, but also the relative configuration. Their ECD (electric circular dichroism) spectra suggested the absolute configurations, which were confirmed with time-dependent DFT calculations employing BHandHLYP/TZVP. The ECD calculations for other stereoisomers yielded obviously different spectral profiles, thus confirming the relative structures of 1 and 3. (Chemical Equation Presented) © 2015 The American Chemical Society and American Society of Pharmacognosy.
Freeman F.,University of California at Irvine |
Bui A.,University of California at Irvine |
Dinh L.,University of California at Irvine |
Hehre W.J.,University of California at Irvine |
Hehre W.J.,Wavefunction Inc.
Journal of Physical Chemistry A | Year: 2012
Structural features of hydrogen thioperoxide (oxadisulfane, H-S-O-H) and of alkanesulfenic acids (R-S-O-H; R = CH3, CH2CH 3, CH2CH2CH3, CH(CH 3)2, C(CH3)3, CF3, CCl3) and the mechanisms of their dehydrative cyclocondensation to the respective sulfinothioic acid (H-(S=O)-S-H) and alkyl alkanethiosulfinates (R-(S=O)-S-R) have been studied using coupled cluster theory with single and double and perturbative triple excitations [CCSD(T)] and quadratic configuration interaction with single and double and perturbative triple excitations [QCISD(T)] with the cc-pVDZ basis set and also using second-order Møller-Plesset perturbation theory (MP2) and the hybrid density functionals B3LYP, B3PW91, and PBE1PBE with the 6-311+G(d,p) basis set. The concerted cyclodehydration mechanisms include cyclic five-center transition states with relatively long distance sulfur-sulfur bonding interactions. Attractive and repulsive nonbonding interactions are predicted in the sulfenic acids, transition states, and thiosulfinates. In the alkyl alkanethiosulfinates attractive cyclic C-H - - O=S nonbonding interactions are predicted. CCSD(T) and QCISD(T) predict similar values for the relative energies and CCSD(T) predicts the barrier to the cyclocondensation of H-S-O-H to sulfinothioic acid (H-(S=O)-S-H) to be 41.8 kcal/mol, and barriers in the range of 37.5 to 39.6 kcal/mol are predicted for the condensation of alkanesulfenic acids to alkyl alkanethiosulfinates. © 2012 American Chemical Society.
PubMed | Wavefunction Inc.
Type: Journal Article | Journal: The journal of physical chemistry. A | Year: 2010
An efficient procedure has been devised for calculating heats of formation of uncharged, closed-shell molecules comprising H, C, N, O, F, S, Cl, and Br. Known as T1, it follows the G3(MP2) recipe, by substituting an HF/6-31G* for the MP2/6-31G* geometry, eliminating both the HF/6-31G* frequency and QCISD(T)/6-31G* energy and approximating the MP2/G3MP2large energy using dual basis set RI-MP2 techniques. Taken together, these changes reduce computation time by 2-3 orders of magnitude. Atom counts, Mulliken bond orders, and HF/6-31G* and RI-MP2 energies are introduced as variables in a linear regression fit to a set of 1126 G3(MP2) heats of formation. The T1 procedure reproduces these values with mean absolute and rms errors of 1.8 and 2.5 kJ/mol, respectively. It reproduces experimental heats of formation for a set of 1805 diverse organic molecules from the NIST thermochemical database with mean absolute and rms errors of 8.5 and 11.5 kJ/mol, respectively. Heats of formation of flexible molecules have been approximated by the heats of formation of their lowest-energy conformer as given by the T1 recipe. This has been identified by examining all conformers for molecules with fewer than 100 conformers and by examining a random sample of 100 conformers for molecules with more than 100 conformers. While this approximation necessarily yields heats of formation that are too negative, the error for typical organic molecules with less than 10 degrees of conformational freedom (several thousand conformers) is <2-3 kJ/mol. T1 heats of formation have been used to calculate energy differences for a variety of structural, positional, and stereoisomers, as well as energy differences between conformers in a variety of simple acyclic and cyclic molecules for which reliable experimental data are available. In terms of both overall error and errors for individual systems, T1 provides a better account of the experimental thermochemistry than any practical quantum chemical method that we have previously examined. A database of approximately 40,000 T1 calculations for both rigid and flexible organic molecules has been produced and is available as part of the Spartan Molecular Database (SMD) in the current version of the Spartan electronic structure program (Spartan08). (A subset of approximately 5000 molecules is provided as part of the standard release, and the full T1 database can be licensed.). This collection differs from the other components of SMD in that the lowest-energy conformation for each molecule has been assigned using a high-level quantum chemical method and not molecular mechanics. Thus, it is not only a source of high-quality calculated heats of formation for organic molecules but also a source of conformational preferences.