Search

The collisional energy transfer dynamics relevant to the unimolecular kinetics of linear, branched, and cyclic hydrocarbons, including both radicals and saturated and unsaturated molecules, in atomic and diatomic baths is studied via classical trajectories. A set of full-dimensional potential energy surfaces (PESs) suitable for efficient trajectory simulations involving large hydrocarbons (C x H y ) colliding with any of seven baths (M = He, Ne, Ar, Kr, H 2 , N 2 , O 2 ) is validated against direct dynamics calculations for two small systems. The PESs are then used to calculate Lennard-Jones collision parameters, and a general rule for calculating these parameters based only on the number of carbon atoms and the bath gas is obtained. Next, the PESs are used to calculate low-order moments of the collisional energy transfer function relevant to low-pressure-limit unimolecular kinetics for a total of 266 systems (38 unimolecular reactants in 7 baths), with a focus on the average angular momentum and total energy transferred in deactivating collisions. These moments are used to quantify the relative rotational and total collision efficiencies of the 7 baths for the various hydrocarbon reactants. Trends in the collision efficiencies with respect to the chemical structures of the hydrocarbon reactants are discussed.

International audience; 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 Moller-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 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.

We report total and differential cross sections for photodetachment from negative ions using Dyson orbitals calculated from equation-of-motion coupled-cluster wave functions and free wave description of the detached electron. The energy dependence of the cross sections is reproduced well, however, the accuracy of absolute values varies. For F(-), C(-), NH(2)(-), and H(-), the calculated cross sections are within the error bars from the experimental values, whereas the errors for Li(-) and OH(-) are about 20%. The largest errors are observed for O(-) and O(2)(-) for which the calculated cross sections differ from the experimental ones by factors of 3 and 2, respectively. Calculated anisotropy parameters for atomic anions exhibit too slow decrease, which suggests that the diffuseness of the computed Dyson orbitals is underestimated. Moreover, in the asymptotic region, the orbitals exhibit artifactual oscillations probably due to the limitations of Gaussian basis sets. The analysis of the trends in the experimental anisotropy parameters suggests that the interaction of the detached electron with the core, which is neglected in the present model, is important.

Implementation of Dyson orbitals for coupled-cluster and equation-of-motion coupled-cluster wave functions with single and double substitutions is described and demonstrated by examples. Both ionizations from the ground and electronically excited states are considered. Dyson orbitals are necessary for calculating electronic factors of angular distributions of photoelectrons, Compton profiles, electron momentum spectra, etc, and can be interpreted as states of the leaving electron. Formally, Dyson orbitals represent the overlap between an initial N-electron wave function and the N1 electron wave function of the corresponding ionized system. For the ground state ionization, Dyson orbitals are often similar to the corresponding Hartree-Fock molecular orbitals MOs; however, for ionization from electronically excited states Dyson orbitals include contributions from several MOs and their shapes are more complex. The theory is applied to calculating the Dyson orbitals for ionization of formaldehyde from the ground and electronically excited states. Partial-wave analysis is employed to compute the probabilities to find the ejected electron in different angular momentum states using the freestanding and Coulomb wave representations of the ionized electron. Rydberg states are shown to yield higher angular momentum electrons, as compared to valence states of the same symmetry. Likewise, faster photoelectrons are most likely to have higher angular momentum. © 2007 American Institute of Physics. DOI: 10.1063/1.2805393