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 chemical bond in the N(2) molecule is analyzed from the perspective of the quantum mechanical interference effect by means of the recently developed generalized product function energy partitioning (GPF-EP) scheme. The analysis is carried out at the GVB-PP and SC levels, which constitute interpretable independent particle models, while ensuring the correct dissociation behavior for the molecule. The results suggest that some current ideas concerning the bond in the N(2) molecule should be revised. It is shown that, in the absence of the interference effect, there is no chemical bond in the N(2) molecule. The influence of the basis set on the energy partitioning is also evaluated. The interference contributions to the energy are substantially less sensitive to the choice of the basis set than the reference energy, making the investigation of the relative importance of inteference effects in larger systems feasible.

The generalized product function energy partitioning (GPF-EP) method has been applied to investigate the nature of the chemical bond and the origin of the inverted dipole moment of the BF molecule. The calculations were carried out with GPF wave functions treating all of the core electrons as a single Hartree-Fock group and the valence electrons at the generalized valence bond perfect-pairing (GVB-PP) or full GVB levels, with the cc-pVTZ basis set. The results show that the chemical structure of both X (1)Σ(+) and a (3)Π states is composed of a single bond. The lower dissociation energy of the excited state is attributed to a stabilizing intraatomic singlet coupling involving the B 2sp-like lobe orbitals after bond dissociation. An increase of electron density on the B atom caused by the reorientation of the boron 2sp-like lobe orbitals is identified as the main responsible effect for the electric dipole inversion in the ground state of BF. Finally, it is shown that π back-bonding from fluorine to boron plays a minor role in the electron density displacement to the bonding region in both states. Moreover, this effect is associated with changes in the quasi-classical component of the electron density only and does not contribute to covalency in either of the states. Therefore, at least for the case of the BF molecule, the term back-bonding is misleading, since it does not contribute to the bond formation.

The nature of the chemical bond in 1,3-butadiene is analyzed by applying the recently developed generalized product function energy partitioning (GPF-EP) scheme, which allows the calculation of the quantum mechanical interference contribution to the energy in a meaningful and intuitive fashion. The method is applied to investigate the breakage of the middle C-C bond, and the rotation along the torsional angle defined by the carbon atoms. A comparison between bonding in ethylene and butadiene is also performed. It is shown that bond delocalization plays no role in the properties of a conjugated molecule and that existing alternative explanations should be revisited.

Extended quantum chemical calculations were performed for the tetracene dimer to provide benchmark results, analyze the excimer survival process, and explore the possibility of using long-range-corrected (LC) time-dependent (TD) second-order density functional tight-biding (DFTB2) for this system.Ground-and first-excited-states optimized geometries, vertical excitations at relevant minima, and intermonomer displacement potential energy curves (PECs) were calculated for these purposes. Groundstate geometries were optimized with the scaled-opposite-spin (SOS) second-order Møller-Plesset perturbation theory (MP2) and LC-DFT (density functional theory) and LC-DFTB2 levels. Excited-state geometries were optimized with SOS-ADC(2) (algebraic diagrammatic construction to second-order) and the time-dependent approaches for the latter two methods. Vertical excitations and PECs were 1 This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in D. C. A. Valente et al. J. Chem. Phys.