Unrestricted Hartree-Fock instabilities in semiempirical CNDO/S and INDO/S calculations of spin-spin coupling constants

It is shown that non-convergent calculations of the Fermi contact term of spin-spin coupling constants within the self-consistent and finite perturbation schemes used to solve the coupled Hartree-Fock equations, are originated in non-singlet Hartree-Fock instabilities of the closed-shell restricted Hartree-Fock wavefunction. In CNDO/S and INDO/S wavefunctions, where the electronic system response has been successfully reproduced, all investigated molecules containing MOs were found to be unstable. Results of spin-spin coupling constants are given and compared with experimental as well as FP and SOS INDO values.Part of a Ph.D. thesis (G.E.S.) to be presented to the University of Buenos Aires.Comisión de Investigaciones Científicas (CIC, Pcia. de Bs. As.) fellow.