Restricted pair interaction, computer

The molecular and electronic structures of cyclic disulfide cation radicals of 1,2-dithietane 6 and 1,2-dithiete 7, and radical cations of 1,2-dithiolane 2 (2a-c represent stable conformations determined in terms of the symmetry restriction of Cs, Cz, and Czv), with emphasis on the nature of a two-center three-electron (Zc-ie) sulfur-sulfur bond have been examined by ab initio molecular orbital (MO) calculations <1997JMT(418)171>. Unrestricted Hartree-Fock (UHF)/ MIDI-4(d) computations showed that this bond in organodisulfide radical cation 2 is shorter in comparison to 1,2-dithiolane 2 and possesses partial Jt-bond character (structure A), as previously implied by electron spin resonance (ESR) spectroscopy <1982JA2318>, which correlates best with the form as the most favorable conformation of the cation radical 2. Contrary to the repulsive S-S interaction in the parent 1,2-dithiolane arising from the lone pairs of electrons, the hemi-7t-bond formed by one-electron oxidation should stabilize the five-membered ring of 2, or, for example, a similar cation radical of LA 3 which is involved in diverse biochemical reactions. 

The incorporation of electron correlation effects in a relativistic framework is considered. Three post Hartree-Fock methods are outlined after an introduction that defines the second quantized Dirac-Coulomb-Breit Hamiltonian in the no-pair approximation. Aspects that are considered are the approximations possible within the 4-component framework and the relation of these to other relativistic methods. The possibility of employing Kramers restricted algorithms in the Configuration Interaction and the Coupled Cluster methods are discussed to provide a link to non-relativistic methods and implementations thereof. It is shown how molecular symmetry can be used to make computations more efficient. 

To exactly solve Eq. 1 the iterative Hartree-Fock (HF) procedure is the most widely used. To obtain exact results, a complete (i.e. infinite) basis set and a full Configuration Interactions (Cl) method would be needed. This is unattainable so a finite basis set is used. Methods that evaluate all the integrals needed to solve Eq. 1 are called ab initio methods. The HF method does not consider the instantaneous interaction within pairs of electrons but only an average force. Not considering this instantaneous interaction (correlation energy) may lead to severe errors. Different methods exist to include the correlation energy, the already cited Cl procedure (not at the usually unattainable full level) being the most evident way to overcome this problem. These methods, called post-HF, are quite costly in computational time so that they are restricted to small systems, well beyond the supramolecular level studied here. 

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