Spin-coupled Valence Bond Theory Research Articles

Ab initio calculation of the zero-field splitting parameter D of benzene and naphthalene

The zero-field splitting (zfs) parameter D of benzene and naphthalene in their lowest excited triplet state has been calculated within the framework of spin-coupled valence bond theory. The predicted parameters of these molecules turn out to be 0.168 cm−1 and 0.124 cm−1, respectively, in reasonably good agreement with the observed values of 0.159 cm−1 and 0.101 cm−1.

Study of the electronic states of the allyl radical using spin-coupled valence bond theory

The spin-coupled valence bond method is used to study the doublet valence excited states and the n=3 Rydberg excited states of the allyl radical below the first ionization potential 8.13 eV. The calculations included three π electrons in the active space. Our results are close to those resulting from the most extensive MO-CI calculations reported to date. The spin-coupled VB approach has an advantage of providing a compact description of the various states.

Spin coupled valence bond theory of van der Waals systems: application to LiH … He

Spin coupled valence bond (SCVB) theory is applied to the study of weakly interacting systems. This ab initio approach is fully variational and it avoids carefully one of the main difficulties which plagues conventional molecular orbital based supermolecule calculations, namely the basis set superposition error. In addition, it proves possible to obtain a physically sound separation of the interaction energy into components such as the electrostatic, exchange, induction/polarization and dispersion contributions, in the same spirit as approaches based on perturbation theory. The SCVB approach is applied to the LiH … He interaction, with the LiH fragment fixed at its equilibrium geometry. The results highlight the accuracy of the method, relative to previous supermolecule studies. In agreement with previous work, the absolute minimum is found for the collinear approach of the helium atom to the lithium end of LiH. A small shallow secondary minimum, which might influence the dynamics at very low collision en...

Study of the electronic states of the benzene molecule using spin-coupled valence bond theory

The spin-coupled VB method is used to study all the singlet and triplet valence excited states, as well as the n=3,4 singlet and triplet Rydberg states of benzene below the first ionization potential at 9.25 eV. The valence excited states are classified in an obvious physical way into covalent or ionic states, from which it follows at once that covalent states are well described using the approximation of σ/π separation and a frozen σ core, whereas the error in the computed transition energies to the ionic states is much larger and these states require additional σ/π correlation for their proper description. The Rydberg states are very well-described, provided that a suitable σ core, derived from a calculation on the C6H+6 ion, is used. The numerical accuracy of the final results for the transition energies is at least the same as that given by the largest MO-CI- or CASSCF-CI-based methods reported to date. The spin-coupled VB approach has the obvious advantage in providing a compact and clear picture of the various states.

Applications of spin-coupled valence bond theory

Applications of spin-coupled valence bond theory

Description of molecular electronic structure using spincoupled valence bond theory

Description of molecular electronic structure using spincoupled valence bond theory

The Spin-coupled Approach to Electronic Structure

Abstract Recent applications of spin-coupled theory are described, in which accuracy is combined with a clear physical picture of the behaviour of correlated electrons in molecules. Results are presented for gas phase ionic systems (H2O++ and CH4+), to indicate the level of accuracy that can be achieved. The physical pictures that emerge are illustrated by reference to a variety of systems, including CH2, C10H8, CH2N2, VH and Li4. Opportunities for the future application of spin-coupled valence bond theory to the calculation of interionic potentials in solids are stressed, including techniques analogous to those that have been used to study intermolecular forces.

Spin-coupled valence bond study of the lithium hydride anion

The anion of lithium hydride has been studied using spin-coupled valence bond theory. An accurate potential-energy curve has been calculated for the ground state of LiH– using a very compact wavefunction consisting of just 55 spatial configurations. This wavefunction is dominated by the spin-coupled configuration over the entire range of R considered. The first four spin-coupled orbitals for LiH– are remarkable similar to those for the neutral molecule. The ‘extra’ electron occupies a diffuse non-bonding s–p hybrid orbital localized on lithium and directed away from the hydrogen atom. Since this fifth orbital is spatially far removed from the others, and has small overlap integrals with them, the correlation correction to the electron affinity is small.

Spin-coupled VB study of the di-cations of methane, ammonia and water

The di-cations of methane, ammonia and water are studied using spincoupled valence bond theory with a compact set of 187 spatial configurations. Vertical excitation energies are reported at the equilibrium geometries of the neutral molecules for a large number of singlet and triplet states over an energy range of more than 20eV. There is very favourable agreement with previous theoretical and experimental work, where available, and ionization potentials are predicted for some symmetries that have not been considered previously.

Spin-coupled valence bond theory

Abstract In the spin-coupled description of molecular electronic structure, an N-electron system is described by N distinct—but non-orthogonal—orbitals, whose spins are coupled to the required resultant S in all possible ways. The coefficients of the basis functions comprising the orbitals and the coefficients of the different spin functions are fully optimized. The orbitals are frequently highly localized, and hence the model incorporates considerable electron correlation while retaining a high degree of visuality. The spin-coupled wave function is refined by non-orthogonal configuration interaction, and the final wave functions are of high quality but very compact. The various aspects of this theory are illustrated by a series of examples of increasing complexity: the H2 molecule, the BeH molecule, the 3B1 and lA1 states of CH2 and the cycloaddition of CH2 to ethenes, the 7t-electron system of benzene, and diazomethane (CH2N2). The results provide clear descriptions of the electronic structure and the associated processes. In benzene, the six π orbitals are highly localized, with far-reaching implications for the description of aromatic systems. The case of diazomethane shows that the central N atom takes part in fiv. electron-pair bonds, and the same is true for a series of molecules such as N2O, Hcno, NO2, and CH2NHO (nitrone), whose structures have long caused problems in valency theory.

Study of the spectrum and decay of the doubly charged water ion using spin-coupled valence bond theory

Potential energy surfaces are computed for several low-lying singlet and triplet states of H2O++ using spin-coupled valence bond theory. Wide regions of the surfaces are investigated including all the dissociation processes to OH++H+ in different states, to O+H++H+ and to O++H++H. The correlation of all these states is very straightforward. The final wave functions are very compact (187 spatial configurations) and the electronic structure of the various states is interpreted directly in terms of the spin-coupled orbitals. This makes possible a completely new description of the ion–ion coincidence (PIPICO), Auger, and double charge transfer spectra of doubly ionized water.

The spin-coupled valence bond theory of molecular electronic structure. I. Basic theory and application to the 2 Σ + states of BeH

The spin-coupled valence bond theory of molecular electronic structure is developed, according to which the single configuration spin-coupled theory is reformulated so as to yield both ground and excited orbitals. These orbitals are subsequently used to generate v.b. structures, the Hamiltonian matrix of which is diagonalized as in the conventional v.b. method. The fundamental feature of the excited spin-coupled orbitals is that, except those with the highest energy, they retain the characteristic distorted atomic form of the ground state orbitals, and correspondingly possess negative orbital energies. This leads to compact and rapidly convergent wavefunctions for the ground and lower-lying excited states, thus overcoming one of the basic drawbacks of the original v.b. theory. The theory is applied to the 2 ∑ + states of BeH by using 53, 71 and 80 structures of this kind. Very good convergence is found for the lowest six states, and the total energy of the ground state is below that given by a very large m.o.c.i. calculation. The present theory is thus a powerful and flexible alternative to m.o.c.i. calculations but using about an order of magnitude fewer functions.