By introducing an electron bath that represents the chemical environment in which a chemical species is immersed, and by making use of the second-order Taylor series expansions of the energy as a function of the number of electrons in the intervals between N - 1 and N, and N and N + 1, we show that the electrodonating (omega-) and the electroaccepting (omega+) powers may be defined as omega-/+ = (mu-/+)2/2eta-/+, where mu-/+ are the chemical potentials and eta-/+ are the chemical hardnesses, in their corresponding intervals. Approximate expressions for omega- and omega+ in terms of the ionization potential I and the electron affinity A are established by assuming that eta- = eta+ = eta = mu+ - mu-. The functions omega-/+(r) = omega-/+f -/+(r), where f -/+(r) are the directional Fukui functions, derived from a functional Taylor series for the energy functional truncated at second order, represent the local electrodonating and electroaccepting powers.

with details of the nature of the infringement. We will investigate the claim and if justified, we will take the appropriate steps.

Density functional calculations are reported for charge-transfer complexes (CT), also called electron donor−acceptor systems, formed from ethylene or ammonia interacting with a halogen molecule (C2H4···X2, NH3···X2, X = F, Cl, Br, and I). In all cases, the local density approximation provides a strong overestimation of the intermolecular interaction. The generalized gradient approximation moves the results in the right direction but, in general, not nearly far enough; large errors remain. We attribute the problem to the too rapid asymptotic decay of the exchange−correlation potential associated with the imperfect cancellation of the self-interaction. This breakdown of the potential is reflected in a set of incorrect eigenvalues (orbital electronegativities) that play a crucial role in governing the charge transfer and, hence, the interaction energy. The inclusion of some Hartree−Fock exchange using hybrid methods provides a large improvement, and the parameters related to the intermolecular interaction for the so-called half-and-half potential are in very good agreement with those obtained through second-order Møller−Plesset calculations and with available experimental data. However, the more widely used three-parameter, B3LYP, functional does not perform well; the hybrid methods are not a panacea.

We summarize our contributions on the quest of new planar tetracoordinate carbon entities (new carbon molecules with exotic chemical structures and strange bonding schemes). We give special emphasis on the rationalization why in this type of molecules the planar configuration is favored over the tetrahedral one. We will concentrate on the latter and will show that molecules containing planar tetracoordinate carbons have a stabilizing system of delocalized electrons, which shows similar properties as systems in aromatic molecules.

The development of analytic-gradient methodology for excited states within conventional timedependent density-functional theory (TDDFT) would seem to offer a relatively inexpensive alternative to better established quantum-chemical approaches for the modeling of photochemical reactions. However, even though TDDFT is formally exact, practical calculations involve the use of approximate functionals, in particular the TDDFT adiabatic approximation, whose use in photochemical applications must be further validated. Here, we investigate the prototypical case of the symmetric CC ring opening of oxirane. We demonstrate by direct comparison with the results of high-quality quantum Monte Carlo calculations that, far from being an approximation on TDDFT, the TammDancoff approximation (TDA) is a practical necessity for avoiding triplet instabilities and singlet near instabilities, thus helping maintain energetically reasonable excited-state potential energy surfaces during bond breaking. Other difficulties one would encounter in modeling oxirane photodynamics are pointed out but none of these is likely to prevent a qualitatively correct TDDFT/TDA description of photochemistry in this prototypical molecule.