UNRELIABLE DESCRIPTION OF DIANIONS BY USING STANDARD DIFFUSE FUNCTIONS (6-31 + G) WITH THE RHF-LCAO METHOD - THE CASE OF THE RELATIVE STABILITY OF Y-SHAPED VERSUS LINEAR-CONJUGATED DIANIONS

The effect of the exponent of diffuse functions augmenting the 6-31G split-valence basis set on the relative stability of Y-shaped versus linear conjugated dianions has been investigated within the restricted Hartree-Fock (RHF) approach. The use of standard diffuse functions in the STO 6-31 + G basis set provides meaningless values of structural parameters, charge distribution, and Y/linear stability. Dianions are usually computed to lie energetically above the ground state of corresponding neutrals so the electron charge in the HOMO tends to be mainly localized at diffuse functions. This localization is influenced by the energy of the two highest MOs. The relative stability of structural isomeric dianions can be computed reliably without including diffuse functions in the basis set since addition of moderately diffuse functions does not affect its value significantly.