Introduction to Valence Bond Theory

As a classical theory of chemical bonding, valence bond theory was once widely used in chemical research and played an important role in the understanding of molecular configuration and the explanation of chemical reaction mechanisms. Nowadays, although valence bond theory is still involved in basic chemistry education, chemists mainly use molecular orbital theory to explain chemical phenomena and understand reaction mechanisms in their specific research work.

Definitions and formulas are always daunting for beginners. Unless necessary, this course tries to abandon boring definitional expositions and lengthy and complicated formulas in its content organization, and tries to demonstrate the concepts and characteristics of valence bond theory and its applications in chemical research in simple language with concrete examples.

In addition to theoretical learning, practice is also very important. In this course, participants will learn how to perform valence bond calculations using the XMVB software on the XACS cloud computing platform, and will be provided with post-course exercises for further thinking and experimentation. We hope that through this course, participants will understand the basic concepts and characteristics of valence bond theory, have the ability to think in terms of valence bond theory, and learn basic software applications.

In addition to the content of this course, we recommend the following literature and books as extended reading material for those who have the capacity to do so:

1. Shaik, S.; Philippe C., H. A Chemist’s Guide to Valence Bond Theory; John Wiley & Sons, Inc., 2007.

2. Wu, W.; Su, P.; Shaik, S.; Hiberty, P. C. Classical Valence Bond Approach by Modern Methods. Chemical Reviews 2011, 111 (11), 7557–7593.

3. Ying, F.; Zhou, C.; Shurki, A.; Danovich, D.; Stuyver, T.; Braïda, B.; Wu, W. A Tutorial on XMVB. In Comprehensive Computational Cheimstry; Yanez, M., Boyd, R. J., Eds.; Elsevier, 2022; Vol. 1, pp 276–310.

4. Zhou, C.; Ying, F.; Wu, W.; Su, P. Valence Bond Methods for Molecules in Solution. In Reference Module in Chemistry, Molecular Sciences and Chemical Engineering; Yanez, M., Boyd, R. J., Eds.; Elsevier, 2022; Vol. 1, pp 403–423.

Table of Content

• 1. What is Valence Bond (VB) Theory?
  • 1.1. Introduction to Classical Valence Bond Theory
  • 1.2. Characteristics of Valence Bond (VB) Theory: Advantages and Limitations
  • 1.3. Common Misconceptions about Valence Bond Theory
  • 1.4. When Should Valence Bond Theory Be Used?
• 2. Valence Bond Structures
  • 2.1. Active Space
  • 2.2. Rumer’s Rule
    • 2.2.1. Closed-shell case
    • 2.2.2. Open-shell case
  • 2.3. How to interpret valence bond structures in XMVB?
  • 2.4. Which Valence Bond Structures Are More Important?
• 3. Valence Bond Orbitals
  • 3.1. Differences and Connections Between VB and MO Orbitals
  • 3.2. Different Types of VB Orbitals
    • 3.2.1. Hybrid Atomic Orbital (HAO)
    • 3.2.2. Coulson-Fischer Orbital (CFO)
    • 3.2.3. Overlap Enhanced Orbital (OEO)
  • 3.3. Methods for Constructing Valence Bond Orbitals in XMVB
    • 3.3.1. The Simplest: Directly Specifying Basis Functions
    • 3.3.2. Constructing Valence Bond Orbitals by Specifying Atoms
    • 3.3.3. Constructing Valence Bond Orbitals Using the Symmetry of Basis Functions
• 4. Bond Dissociation Energy
  • 4.1. Definition of Bond Dissociation Energy
  • 4.2. How to Compute Bond Energy?
• 5. Resonance Energy
  • 5.1. Definition of Resonance Energy
  • 5.2. How to Calculate Resonance Energy?
  • 5.3. Applications of Resonance Energy
    • 5.3.1. Charge-Shift Bond
    • 5.3.2. The Essence of Aromaticity
• 6. Excited States
  • 6.1. Different Pictures of Excited States in MO and VB Theories
  • 6.2. How to Calculate Excited States in XMVB
    • 6.2.1. Obtaining Specific Excited States
    • 6.2.2. State-Averaged Calculations
• 7. Valence Bond Theory in Chemical Reactions
  • 7.1. What is a Transition State?
  • 7.2. Computing Transition State Wave Functions with XMVB
  • 7.3. Valence Bond State Correlation Diagram (VBSCD)
• 8. Valence Bond Theory and Diabatic States
  • 8.1. What is an adiabatic state? What is a diabatic state?
  • 8.2. Why use valence bond theory to compute diabatic states?
  • 8.3. How to compute diabatic states in XMVB?
• 9. How to Provide a More Effective Initial Guess
  • 9.1. Several Different Schemes for Initial Guess
    • 9.1.1. GUESS=AUTO
    • 9.1.2. GUESS=READ
    • 9.1.3. GUESS=MO/NBO
    • 9.1.4. GUESS=UNIT
  • 9.2. How to Choose an Appropriate Initial Guess Scheme?