The Representation of the Chemical Bond in Quantum Chemical Calculations

The mathematical representation of chemical bonds in molecules is discussed. The molecule is viewed as a system of weakly interacting chemical bonds. The intrabond problems and the averaged electrostatic interbond interactions are handled at the . zeroth order, while the small interbond delocalization and dispersion effects are taken into account on the basis of the perturbation theory. A special diagrammatic technique is applied for obtaining delocalization corrections for strictly localized orbitals. A general second quantized theory is discussed in which the chemical bonds are identified with two-electron local bond structures characterized by composite-particle creation operators showing Bose-type commutation rules. This approach accounts for intrabond correlation already at the zeroth order.