Chemical Bonding in Many Electron Molecules

Chemical bonding is the stabilization of a composite molecular system caused by different interactions in and between the subsystems, among the strong kinds of bonding is covalent bonding especially important. Characteristic for covalent bonding are small atom groups with short distances between the involved atoms, indicating that covalent bonding is essentially a local effect, according to Lewis, this is caused by shared electron pairs. However, the energetic stabilization is an approximately additive one-electron effect, as was shown by Ruedenberg and coworkers. In systems composed of many-electron subsystems, the fermionic character of the electrons determines the structure of the electron distribution in a subsystem, and it is decisive for the local interactions between the subsystems. Especially important is the Pauli exclusion principle (PEP), which directs the relative positions of identical electrons. Spin and charge rearrangements are of utmost importance for chemical bonding. Quantum chemical methods like CASSCF (complete active space SCF), also called FORS (fully optimized reaction space), are made to cover all such processes. The standard building blocks of CASSCF wave functions are delocalized molecular orbitals, which cannot display local effects. OVB (orthogonal valence bond) is a method to analyze CASSCF wave functions and to reveal local processes that are responsible for both the energetic aspects of bonding and the spatial structure of the stabilized system. This is shown by analyzing dissociation of ethene, disilene, and silaethene, and the corresponding reverse reactions. Aspects of diabaticity of the reactions and entanglement of subsystems are discussed.