Hydrogen molecule minimal basis description

With respect to correlation, the behaviour of the hydrogen molecule studied in a subminimal FSGO basis set is still more striking than the one observed in a minimal basis set. By symmetry arguments, the single FSGO which describes the electron pair of the hydrogen molecme is centred at the middle of the H-H bond. As the intemuclear distance increases and ultimately when the molecule dissociates, such a description would lead to a physical nonsense. Indeed, at the dissociation limit, this would correspond to two protons (2H ) and an isolated pair of electrons (2e ). 

Single determinant ab initio LCAO-SCF molecular orbital theory is used throughout this study (6,7), Molecular geometries were optimized with the minimal STO-3G basis set (8,9), and where possible the energy of the final structure was recalculated with the extended 4-31G basis set (9,10), Such a procedure has been shown to provide a reasonable description of the structures and energies of neutral organic molecules (11) and carbocations (12), The potential surface for the concerted elimination of hydrogen chloride from ethyl chloride also has been studied successfully (13) with this technique. 

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