Split-valence basis sets orbital energy calculations using

The different struetures and transitions states of interest in the neutral and negative ion reaetions are represented in Fig. 2. A first approach was done at the SCF level, using the split-valence 4-3IG basis set. In order to provide a better estimation of the energy differences implied in this reaction schemes, extensive calculations have been performed at the MP2 level of theory using the 6-311++G basis set which contains the diffuse orbitals necessary to quantitatively describe the negative ions. 

Molecular orbital calculations at various levels of approximation have been applied to both furazans and furoxans. Ab initio procedures using minimal (STO-3G) and split valence (3-21G) basis sets have been used to determine bond orders, total energies, ionization potentials, and dipole moments for the parent furazan and furoxan, and several derivatives (88JCS(P2)66l) the calculated molecular geometries (3-21G) are compared in Table 3 with those obtained experimentally. 

The 4-3IG basis set is not exactly a double zeta basis since only the valence functions are doubled and a single function is still used for each inner shell orbital. It may be termed a split valence shell basis set. The inner shells contribute little to most chemical properties and usually vary only slightly from molecule to molecule. Not splitting the inner shell functions has some effect on the total energy, but little effect on dipole moments, valence ionization potentials, charge densities, dissociation energies, and most other calculated quantities of chemical interest. The 4-3IG basis thus consists of 2 functions for H and He, 9 functions for Li to Ne, 13 functions for Na to Ar,..., etc. For hydrogen the contractions are... 

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