Group-theoretical analysis of carbonium ion rearrangements: The barbaralyl cation

The 9-barbaralyl and related C9H9+ carbocations — A QTAIM-DI-VISAB computational study

QTAIM-DI-VISAB analyses were used to characterize the bonding of the 9-barbaralyl cation, related C9H9+ cations, and rearrangement transition states. These analyses involved obtaining quantum theory of an atom in a molecule (QTAIM) molecular graphs and delocalization indexes (DIs) that were correlated with visualization of the proximities of atomic basins (VISAB). This study provides new insights into the bonding of these species and cements the QTAIM-DI-VISAB analysis as a method of choice for establishing the nature of the bonding in so-called nonclassical carbocations, while obviating the need for dashed-line representations of bonding.

First-principles calculations of zero-field splitting parameters

In this work, an implementation of an approach to calculate the zero-field splitting (ZFS) constants in the framework of ab initio methods such as complete active space self-consistent field, multireference configuration interaction, or spectroscopy oriented configuration interaction is reported. The spin-orbit coupling (SOC) contribution to ZFSs is computed using an accurate multicenter mean-field approximation for the Breit-Pauli Hamiltonian. The SOC parts of ZFS constants are obtained directly after diagonalization of the SOC operator in the basis of a preselected number of roots of the spin-free Hamiltonian. This corresponds to an infinite order treatment of the SOC in terms of perturbation theory. The spin-spin (SS) part is presently estimated in a mean-field fashion and appears to yield results close to the more complete treatments available in the literature. Test calculations for the first- and second-row atoms as well as first-row transition metal atoms and a set of diatomic molecules show accurate results for the SOC part of ZFSs. SS contributions have been found to be relatively small but not negligible (exceeding 1 cm(-1) for oxygen molecule). At least for the systems studied in this work, it is demonstrated that the presented method provides much more accurate estimations for the SOC part of ZFS constants than the emerging density functional theory approaches.

Relativistic configuration interaction study of the electronic spectrum of SnTe and SnTe+

Ab initio based relativistic configuration interaction calculations have been performed to study the electronic spectrum of the heaviest tin chalcogenide and its monopositive ion. Potential energy curves and spectroscopic constants of low-lying states of both species within 7 eV are reported. The ground-state dissociation energies of SnTe and SnTe+ are computed to be 3.48 and 2.50 eV, respectively. The spin-orbit splitting between the two components of the X 2Pi state of SnTe+ is about 3030 cm(-1). Effects of the strong spin-orbit coupling on the potential curves and spectroscopic properties of both the species are investigated in detail. The electric dipole moments of some of the low-lying states of SnTe and SnTe+ are reported. Transition moments of some important spin-allowed and spin-forbidden transitions are calculated from the configuration interaction wave functions. The radiative lifetime of the excited E 1sigma0+(+) state of SnTe is about 39 ns. The X2-X1 transition in SnTe+ is found to be more probable than the similar transition in the lighter ions. The vertical ionization energy of SnTe in the ground state is estimated to be 8.22 eV.

Cooperative versus dispersion effects: What is more important in an associated liquid such as water?

We implemented the quantum cluster equilibrium theory in our postprocessing program PEACEMAKER. This program may be run in conjunction with the very efficient vibrational frequency analysis code SNF and can therefore provide access to all electronic structure programs combined with this program. We applied the quantum cluster equilibrium theory in order to investigate the influence of a wide range of electronic structure models on the description of the liquid state. This investigation revealed much about the relevance of approximations in modern simulations of associated liquids such as water. While it is often claimed that the use of density-functional theory in condensed matter is leading to gravely erroneous results, we found that, contrary to these assertions, the exact exchange functional B3LYP and the gradient-corrected functional BP perform very well in combination with sizable basis sets as compared to second-order Moller-Plesset perturbation theory employing the same basis set. The use of density-functional theory with smaller basis sets does, in fact, lead to better results in the liquid state than the use of second-order Moller-Plesset perturbation theory in combination with these small basis sets. Most importantly, the neglect of cooperative effects disturbs a good description much more evenly if we apply second-order Moller-Plesset perturbation theory in combination with large basis sets than density-functional theory including cooperativity with smaller basis sets or Hartree-Fock using a very small basis set.