The energy difference between the three lowest-lying isomers of C6, the linear 3Σ−g state, and the two ring forms, the benzene structure (1A1g) possessing D6h symmetry and a distorted cyclic form (1A1′, D3h symmetry) have been calculated using various ab initio methods. Variational methods such as multireference configuration interaction (MR-CI) and complete active space second order perturbation treatment (CASPT2) have been applied, as well as perturbational treatments and coupled cluster calculations (CCD). The correlation of all valence shell electrons is found to be important for a balanced description of the isomers of C6. Methods which do not account for higher-order effects appropriately proved to be unsuitable for calculating the energy difference correctly. The results from multireference configuration interaction methods show that the isomers are close in energy with the cyclic forms somewhat lower than the linear form. The ring form possessing D3h symmetry (1A1′) is found to be the lowest-lying structure.

Density functional theory is applied to the calculation ofthe isotropic byperfine coupJing constants in some small molecules.

A recently proposed ab initio method for calculation of shape resonance, which is based on the Feshbach formalism, is applied to low-energy electron scattering of the HCl molecule. A metastable 2 Sigma + state of the HCl- anion is assumed to be responsible for the structures, which have been found experimentally in the excitation cross section. In the authors' approach resonances are described by a metastable anion, the so-called discrete component. This anion is obtained by a stabilization procedure invoking an increase of the electron-nuclei interaction. The use of a configuration interaction procedure for the calculation of the resonant state provides for a description of correlation and polarization effects between target and projectile. The construction of the metastable 2 Sigma + state is discussed in detail as well as the influence of the stabilization on the wavefunction of the molecule and the anion.

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Based on the Feshbach projection-operator formalism a method for calculating angle-integrated and resonant cross sections for vibrational excitation and dissociative attachment of diatomic molecules by low-energy electrons is developed. The method allows for a treatment of the non-local character of the scattering potentials. The basic formulae reduce to a set of linear equations. As a first example the HCl molecule is considered using model potentials. The calculated cross sections are found to be in good agreement with the results of an exact solution of the model problem.