Breathing-orbital valence bond method – a modern valence bond method that includes dynamic correlation
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This account describes the breathing-orbital valence bond (BOVB) method, a modern valence bond method that incorporates differential dynamic correlation associated with the bond making and breaking of a chemical event. The method aims to combine the properties of interpretability and compactness of the classical valence bond method with good accuracy of the energetics. The domain of applicability of the BOVB method is mainly for problems that require interpretation of an electronic wave function in terms of Lewis structures. The method generates all the covalent and ionic Lewis structures that are relevant to the electronic state, and represents each of them by a single valence bond configuration state function. A balanced description of the different Lewis structures is then ensured by allowing each configuration to have its specific set of orbitals during the optimization process. In this framework, the dynamic correlation associated with the breaking or forming of a bond is viewed as the instantaneous adaptation of the orbitals to the electron fluctuation inherent to the bond; hence, the “breathing orbital” characterization. Applications of the BOVB method to a variety of problems are described, for example, two-electron bonds, odd-electron bonds, bonds to transition metals, resonance energies, and diabatic surfaces. In all these applications, the method is shown to provide bonding energies that compare well to accurately calculated or experimental values, despite the extreme compactness of the wave functions.
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