A novel method for investigating the multicenter bonding patterns in molecular systems by means of the so-called Electron Density of Delocalized Bonds (EDDB) is introduced and discussed. The EDDB method combines the concept of Jug's bond-order orbitals and the indirect ("through-bridge") interaction formalism and opens up new opportunities for studying the interplay between different atomic interactions as well as their impact on both local and global resonance stabilization in systems of conjugated bonds. Using several illustrative examples we demonstrate that the EDDB approach allows for a reliable quantitative description of diverse multicenter delocalization phenomena (with special regard to evaluation of the aromatic stabilization in molecular systems) within the framework of a consistent theoretical paradigm.

The recently introduced quadratic (two-electron) valence indices, ionic and covalent, derived from the HartreeFock finite-difference approach, are applied to selected organic and inorganic molecules to demonstrate their utility in monitoring chemical bonding patterns in molecular systems. The indices are defined in terms of differerences between simultaneous probabilities of finding two electrons on specified atoms, calculated from the molecular and separated-atomlimit (SAL) wave functions, respectively, in the UHF approximation. The total quadratic valence number represents the overall number of chemical bonds in the system under consideration; it is interpreted as the molecular expectation value of the difference operator of the molecular and SAL density operators. This interpretation leads to a new set of ionic atomic and diatomic valence components; these modified valence numbers are discussed using the two-orbital model in the UHF scheme. A new procedure is proposed for dividing the one-center contributions to the bond valences; it generates effective bond orders in qood agreement with chemical expectations. The new valence quantities are tested on selected typical molecules and prototype hydrogen-bonded dimers. A more extensive study has been canied out on small-ring propellanes, to examine changes in bond valences between bridgehead atoms in selected systems.

We present a novel density-functional tight-binding (DFTB) parametrization toolkit developed to optimize the parameters of various DFTB models in a fully automatized fashion. The main features of the algorithm, based on the particle swarm optimization technique, are discussed, and a number of initial pilot applications of the developed methodology to molecular and solid systems are presented.

According to recent ab initio calculations, the energy gap between the two oppositely polarized charge-transfer (CT) states of a model pentacene dimer is anomalously large, attaining 0.8 eV. Here we introduce the self-consistent charge field approach to evaluate the electrostatic stabilization energies of the pertinent states in the pentacene crystal represented by a dedicated multiscale model system containing the model dimer as its core. We demonstrate that, contrary to common wisdom, the lower of the two CT states is barely affected by the crystalline environment whereas the upper one undergoes a large red shift. Effectively, embedding of the dimer in the crystal bulk reduces the pertinent splitting by an order of magnitude, because most of the intradimer charge−quadrupole interactions are compensated by similar interactions with surrounding molecules. This resolves the apparent contradiction between the ab initio result obtained for the dimer and the splitting of about 0.04 eV resulting from microelectrostatic calculations for the crystal. ■ INTRODUCTIONThe discovery of spontaneous singlet exciton fission (SF) in solid pentacene prompted widespread interest in the electronic properties of this system, in the recent years triggering a surge of theoretical papers that strive to unravel the underlying physical mechanisms. 1−4 Various methodologies have been invoked for this purpose.No matter how vibronic coupling is treated and how the dynamic aspects of the problem are described, 1−3 the process is ultimately controlled by the energetics of the electronic states involved. Yet, as the small energy gaps between the relevant eigenstates are inevitably sensitive to the details of the wave function, the requisite accuracy is just on the verge of the predictive power of the most sophisticated quantum chemistry tools available to date. By these standards, the pentacene molecule is large. Moreover, in order to describe the fission process, it is necessary to take into account at least the two molecules on which the two emerging triplets are located, which makes a dimer the minimum model system to be considered in this context. At the ab initio level, this creates a numerical problem of formidable complexity, practically ruling out inclusion of any elements of the crystal environment in which the pertinent pair of molecules is in reality embedded.In a number of papers 1−3 focusing on diverse aspects of the fission phenomenon, various versions of the dimer model were adopted. In most cases, parametrization of the model Hamiltonian was in principle based on quantum chemistry calculations but fine-tuned on intuitive grounds to account for simplifications of the model and for potential intrinsic errors of the applied quantum chemistry methods.The paper by Zeng et al. 4 stands out from this collection, representing a consistent ab initio approach at a highly advanced level. However, in view of the accuracy it offers, even minor energy shifts resulting from the simplifications of the underlying model may significantly influen...

Dressed Time-Dependent Density Functional Theory (Maitra et al., J Chem Phys 2004, 120, 5932) is applied to selected linear polyenes. Limits of validity of the approximation are briefly discussed. The implementation strategy is described. Results for the 2(1)B(u) and 2(1)A(g) states of selected linear polyenes are presented and compared with accessible experimental and theoretical results.