The main driving force for the formation of the covalent bond is the quantum-mechanical interference effect among one-electron states, as has been suggested in several works by the use of partition schemes to calculate the interference contributions to the energy. However, due to some difficulties associated with the original approaches, calculations were only carried out for a few, mostly diatomic molecules. In this work, we propose a general approach of partitioning based on generalized product functions with generalized valence bond at the perfect pairing approximation and spin-coupled groups, which should allow the investigation of a broader array of molecules, and hopefully, shed light on the nature of the chemical bond in molecules with unusual chemical features. Among other things, this approach lends itself naturally to the investigation of interference in individual bonds or groups of bonds in a molecule.

The origin of the anomeric effect has remained an open question. After Mo demonstrated that hyperconjugation is not responsible for the anomeric effect [Y. Mo, Nature Chem., 2010, 2, 666.], electrostatic interactions and Pauli repulsions have been at the center of this debate. In this work, the total energies of the most stable rotamers of the equatorial and axial anomers of fluoro, hydroxyl, cyano and amino groups in cyclohexane and 2-substituted tetrahydropyran rings are decomposed into their fundamental kinetic, electrostatic and exchange components. In this partitioning scheme, the differences in the total energies among the most stable rotamers of each anomer correlate very well with the differences in the exchange components, revealing that the anomeric effect has no electrostatic origin. Indeed, the anomeric effect is dominated by the exchange energy. This proposal for the origin of the anomeric effect brings new insights that, once incorporated, may improve qualitative chemical models. Implications of this new proposal for the origin of the anomeric effect on geometric parameters and solvation are also discussed.

Charge transfer, exciton localization and time scales in benzene excimer formation after a S₀–S₁ transition from the parallel-displaced structure were characterized by surface-hopping dynamics.

The absorption and fluorescence spectra of poly(p-phenylenevinylene) (PPV) oligomers with up to seven repeat units were theoretically investigated using the algebraic diagrammatic construction method to second order, ADC(2), combined with the resolution-of-the-identity (RI) approach. The ground and first excited state geometries of the oligomers were fully optimized. Vertical excitation energies and oscillator strengths of the first four transitions were computed. The vibrational broadening of the absorption and fluorescence spectra was studied using a semiclassical nuclear ensemble method. After correcting for basis set and solvent effects, we achieved a balanced description of the absorption and fluorescence spectra by means of the ADC(2) approach. This fact is documented by the computed Stokes shift along the PPV series, which is in good agreement with the experimental values. The experimentally observed band width of the UV absorption and fluorescence spectra is well reproduced by the present simulations showing that the nuclear ensemble generated should be well suitable for consecutive surface hopping dynamics simulations.