Currently Available Ab Initio Valence Bond Computational Methods and Their Principles

Chapter 9 describes VB methods and packages which enable VB calculations, which scale with the electron number as N 4 . There are two main VB approaches. The classical VB approach, VBSCF, utilizes hybrid atomic orbitals (HAOs) localized on a single atom and involves explicit covalent and ionic structures, which undergo optimization to minimize the total electronic energy. VBSCF uses a common set of HAOs for all the VB structures, while BOVB allows the HAOs to be different for different VB structures, hence including static and dynamic correlations. VBSCF is improved by configuration interaction, which conserves the orbital locality, or by adding a Jastrow factor which multiplies the VB determinants. Some alternative approaches, the generalized VB (GVB) and spin-coupled (SC) methods, use formally covalent structures and results in HAOs having small delocalization tails on bonded atoms (GVB-PP), or overlap-enhanced orbitals (OEOs) with tails on many atoms. In addition, the chapter mentions related methods and provides inputs for various calculations.