Breathing‐Orbital Valence Bond: Methods and Applications

This chapter describes the breathing-orbital valence bond method (BOVB) and some of its applications. The improvement of BOVB over the VBSCF method is that BOVB involves dynamic electron correlation by allowing each VB structure to have its specific set of optimized orbitals. Calculations of bonding energies for electron-pair and odd-electron bonds show that BOVB, which utilizes compact wave functions, is as accurate as state-of-the-art MO–CI methods. A quantitative definition of diradical character is proposed and shown to correlate with experimental reaction barriers. In applications to excited states, the BOVB method is used to calculate transition energies for some typical challenging cases, such as the ionic excited state of ethylene, and ionic or diexcited states of ozone and sulfur dioxide. In all cases, the BOVB calculations involve less than ten VB structures and are as accurate as standard MO–CI expansions involving tens or hundreds of millions of configurations.