A valence bond study of the dioxygen molecule

Area of Science:

• Quantum Chemistry
• Theoretical Chemistry
• Computational Chemistry

Background:

• The dioxygen molecule (O2) has historically challenged valence bond (VB) theory since the 1930s.
• Previous VB studies indicated limitations in describing O2's electronic structure and bonding characteristics.

Purpose of the Study:

• To provide a clear valence bond (VB) interpretation of the chemical bonding in the dioxygen molecule.
• To evaluate modern VB computational methods for their accuracy in describing O2.

Main Methods:

• Utilized Valence Bond Self-Consistent Field (VBSCF), Bond-Order Valence Bond (BOVB), and Valence Bond Configuration Interaction (VBCI) methods.
• Computed potential energy curves (PECs) for the ground and excited states of O2.
• Compared VB results with experimental data and sophisticated molecular orbital (MO) based methods like MRCI.

Main Results:

• The VBSCF method provides qualitative insights but lacks quantitative accuracy due to missing dynamic correlation.
• A small hump in the VBSCF potential energy curves for the ground state was observed, consistent with prior studies.
• Higher-level VB methods (BOVB, VBCI) successfully incorporated dynamic correlation, accurately reproducing O2's dissociation energies and physical properties.

Conclusions:

• Accurate quantitative description of dioxygen bonding necessitates VB methods that include dynamic correlation.
• BOVB and VBCI methods offer a reliable and accurate theoretical framework for studying O2 and similar molecules.
• Modern VB computational approaches provide valuable and intuitive understanding of chemical bonding in challenging systems.