Spin-Coupled Generalized Valence Bond Theory: New Perspectives on the Electronic Structure of Molecules and Chemical Bonds

Abstract

Spin-Coupled Generalized Valence Bond (SCGVB) theory provides the foundation for a comprehensive theory of the electronic structure of molecules. SCGVB theory offers a compelling orbital description of the electronic structure of molecules as well as an efficient and effective zero-order wave function for calculations striving for quantitative predictions of molecular structures, energetics and other properties. The orbitals in the SCGVB wave function are usually semi-localized and, for most molecules, can be interpreted using concepts familiar to all chemists (hybrid orbitals, localized bond pair, lone pairs, etc.). SCGVB theory also provides new perspectives on the nature of the bonds in molecules such as C2, Be2 and SF4/SF6. SCGVB theory contributes unparalleled insights into the underlying cause of the first-row anomaly in inorganic chemistry as well as the electronic structure of organic molecules and the electronic mechanisms of organic reactions. The SCGVB wave function accounts for nondynamical correlation effects and, thus, corrects the most serious deficiency in molecular orbital (RHF) wave functions. Dynamical correlation effects, which are critical for quantitative predictions, can be taken into account using the SCGVB wave function as the zero-order wave function for multireference configuration interaction or coupled cluster calculations.* In prior articles in the literature the Spin-Coupled Generalized Valence Bond wave function has usually been referred to as the Spin-Coupled Valence Bond (SCVB) wave function or the full Generalized Valence Bond (full GVB) wave function. These wave functions are identical; to prevent confusion, the authors have adopted the allencompassing SCGVB name. Spin-Coupled Generalized Valence Bond Theory: New Perspectives on the Electronic Structure of Molecules and Chemical Bonds3 and recoupled pair bond dyad. 36 The SCGVB wave function describes molecules in terms of orbitals that are optimized at each geometry, allowing the atomic orbitals to change as a result of molecule formation. In addition to the changes in the orbitals, optimization of the general spin function used in the SCGVB wave function allows the spin function to change from that appropriate for the separated atoms or fragments to that appropriate for the molecule. SCGVB theory addresses a major limitation of RHF theory, providing an excellent zero-order description of molecules that require multiconfiguration descriptions, the making/breaking of bonds, and many chemical reactions and excited states. For most molecules, the SCGVB wave function captures the essential features of the FORS/CASSCF wave functions, with much reduced complexity compared to these latter wave functions.

Multiconfiguration Pair-Density Functional Theory for Transition Metal Silicide Bond Dissociation Energies, Bond Lengths, and State Orderings

“…The partitioning may be done by various schemes, and the present study uses the scheme of the separated-pair (SP) approximation [ 15 , 16 , 17 ]. The SP approximation is like generalized valence bond theory [ 18 ] but without the perfect pairing approximation. Using a precisely formulated method of selecting the active space and its subspaces is important because it eliminates the human judgment issue, which one must remove in order to have a theoretical model chemistry [ 19 ] that can be systematically validated.…”

Non-iterative Method for Constructing Valence Antibonding Molecular Orbitals and a Molecule-adapted Minimum Basis.

“…113 While empty 3d functions on sulfur are needed to form 6 equivalent sp 3 d 2 hybrids, the energetic cost of promoting electrons to the 3d shell is too high for d-orbital participation in the bonding to be chemically important. 46,[114][115][116][117] Rather, the bonding may be thought of as resonance between Lewis structures with 4 covalent S-F bonds, and 2 F − anions, with a formal charge of +2 on S. 118 There are some limitations associated with reducing the union of the localized occupied orbitals and the AB2 antibonding orbitals to a set of MAOs that should be mentioned. First, some conjugated π systems, such as benzene and C 5 H 5 -, present a multiple minimum solution problem for orbital localization methods.…”

“…Thus complete active space (CAS) methods that seek to describe strong correlation require only a description of the valence virtuals. Methods in this class include CASSCF, 2,39-42 spin-coupled valence bond (VB), [43][44][45][46] and approximations such as generalized valence bond (GVB), 47 coupled cluster valence bond (CCVB), [48][49][50] etc.…”

Magnetic Shielding Study of Bonding and Aromaticity in Corannulene and Coronene

“…Magnetically induced currents can be integrated to produce shielding tensors by means of the Biot−Savart law (as explained, for example, in [18]), and the expectation is that the low current density along the Valence bond (VB) theory provides a simple argument against the AWA model [6]: Corannulene has 11 Kekulé resonance structures (see Figure 1a-c), and each of the five spoke bonds is double in five of these resonance structures. At the moment, it is not possible to quantify the relative importance of the Kekulé resonance structures of types K 1 , K 2 and K 3 using modern VB theory, because a spin-coupled generalised VB (SCGVB) [7] calculation with 20 active orbitals is still not computationally feasible (the overall number of unique covalent resonance structures for a 20-electron singlet is 16,796; these include, in addition to the 11 Kekulé resonance structures, a number of less important para-bonded structures, similar to the Dewar structures in benzene, and many other structures with longer pair separations). However, chemical intuition and SCGVB experience with naphthalene [8] suggest that the weight of K 1 , which includes five π sextets, will be higher than the combined weight of structures K 2 , each of which includes three π sextets, followed by the combined weight of structures K 3 , each of which includes two π sextets; this emphasizes the importance of the spoke bonds and lends additional support to the counter-AWA argument.…”